diff --git a/flake8_alfred/__init__.py b/flake8_alfred/__init__.py
new file mode 100644
index 0000000..6b2e22b
--- /dev/null
+++ b/flake8_alfred/__init__.py
@@ -0,0 +1,67 @@
+"""Flake8 plugin emitting warning for obsolete symbols."""
+
+__all__ = ["SymbolsVisitor", "WarnSymbols"]
+
+from ast import AST
+from typing import Any, Dict, Iterator, Optional, Tuple
+
+from pkg_resources import get_distribution
+from .symbols import SymbolsVisitor
+
+
+# Flake8 error type: (line number, column, warning message, caller type)
+FlakeError = Tuple[int, int, str, type]
+
+
+class WarnSymbols:
+    """The flake8 plugin itself."""
+    BANNED: Dict[str, str] = {}
+
+    # The name and version class variables are required by flake8. It also
+    # requires add_options and parse_options for options handling.
+
+    name: str = "warn-symbols"
+    version: str = get_distribution("flake8-alfred").version
+
+    def __init__(self, tree: AST) -> None:
+        self._tree = tree
+
+    @classmethod
+    def add_options(cls, parser: Any) -> None:
+        """Register the --warn-symbols options on parser."""
+        parser.add_option("--warn-symbols", default="", parse_from_config=True)
+
+    # The following method might seem unsafe, but flake8 is designed this way:
+    # The class hold its configuration, and it's the same for every checked
+    # files.
+
+    @classmethod
+    def parse_options(cls, options: Any) -> None:
+        """Load the obsolete symbols into cls.symbols."""
+        lines = options.warn_symbols.splitlines()
+        for line in lines:
+            symbol, _, warning = line.partition("=")
+            cls.BANNED[symbol.strip()] = warning.strip()
+        cls.BANNED.pop("", None)
+
+    def run(self) -> Iterator[FlakeError]:
+        """Run the plugin."""
+        visitor = SymbolsVisitor()
+        for symbol, node in visitor.visit(self._tree):
+            # Get the warning associated to the most specific module name.
+            warning: Optional[str] = None
+            for module in submodules(symbol):
+                warning = self.BANNED.get(module, warning)
+            # If there's no associated warning, it means the symbol is not
+            # deprecated.
+            if warning is None:
+                continue
+            # Otherwise, we yield an error.
+            yield (node.lineno, node.col_offset, f"B1 {warning}", type(self))
+
+
+def submodules(symbol: str) -> Iterator[str]:
+    """submodules("a.b.c") yields "a", then "a.b", then "a.b.c"."""
+    bits = symbol.split(".")
+    for i in range(len(bits)):
+        yield ".".join(bits[:i+1])
diff --git a/flake8_alfred/symbols.py b/flake8_alfred/symbols.py
new file mode 100644
index 0000000..3e5ee76
--- /dev/null
+++ b/flake8_alfred/symbols.py
@@ -0,0 +1,247 @@
+"""Symbols visitor: list all qualified names in a given AST."""
+
+import builtins
+
+from ast import (
+    # Generic AST type
+    AST,
+    # Special
+    arg, arguments, comprehension, ExceptHandler, expr, stmt,
+    # Statements
+    AsyncFunctionDef, ClassDef, FunctionDef, Import, ImportFrom,
+    # Expressions
+    Attribute, DictComp, GeneratorExp, Lambda, ListComp, Name, SetComp,
+    # Expression context
+    Del, Param, Store,
+    # Visitors
+    iter_child_nodes
+)
+
+from typing import (
+    Any, ChainMap as ChainMapT, Iterable, Iterator, Mapping, Optional,
+    Sequence, Tuple, TypeVar, Union
+)
+
+from collections import ChainMap
+from contextlib import contextmanager
+
+from .visitor import Visitor
+
+
+A = TypeVar("A")
+B = TypeVar("B")
+
+Function = Union[AsyncFunctionDef, FunctionDef]
+ScopeT = ChainMapT[str, Optional[str]]
+Symbols = Iterable[Tuple[str, Union[expr, stmt]]]
+UnaryComp = Union[GeneratorExp, ListComp, SetComp]
+
+
+class SymbolsVisitor(Visitor[AST, Symbols]):
+    """SymbolsVisitor.visit yields a pair (qualified_name, node) for all
+    qualified names it finds in the given AST.
+
+    It does handle:
+        - Imports: Importing an obsolete symbol will yield it;
+        - Delete statements: (del obsolete_symbol; obsolete symbol) doesn't
+          yields anything;
+        - Scopes: If you overwrite a symbol in a given function or class scope,
+          it will not be overwriten in outer scopes;
+        - Type annotations.
+
+    We don't support global and nonlocal statements for now, and the assignment
+    operator untrack the symbol if it's on the left hand side, no matter what's
+    on the right hand side.
+    """
+    def __init__(self) -> None:
+        # Load names from the builtins module into self.scopes. If they are
+        # not overwritten, each builtin name is an alias of itself
+        # (self.scopes["print"] == "print")
+        init = dir(builtins)
+        self.scopes: ScopeT = ChainMap(dict(zip(init, init)))
+
+    @contextmanager
+    def scope(self) -> Iterator[Mapping[str, Optional[str]]]:
+        """Context manager that create a new scope (that is, add a mapping into
+        self.scopes) and delete it on exit.
+        """
+        self.scopes = self.scopes.new_child()
+        try:
+            yield self.scopes.maps[0]
+        finally:
+            self.scopes = self.scopes.parents
+
+    def generic_visit(self, node: AST) -> Symbols:
+        """Equivalent to ast.NodeVisitor.visit, except it chains the returned
+        iterators.
+        """
+        for child in iter_child_nodes(node):
+            yield from self.visit(child)
+
+
+# HELPERS
+
+
+def visit_optional(
+        vtor: Visitor[A, Iterable[B]],
+        node: Optional[A]
+) -> Iterable[B]:
+    """Visit an optional node."""
+    if node is not None:
+        yield from vtor.visit(node)
+
+
+def visit_sequence(
+        vtor: Visitor[Any, Iterable[B]],
+        node: Sequence[Any]
+) -> Iterable[B]:
+    """Visit a sequence/list of nodes."""
+    for item in node:
+        yield from vtor.visit(item)
+
+
+# SPECIAL
+
+
+@SymbolsVisitor.on(arg)
+def visit_arg(vtor: SymbolsVisitor, node: arg) -> Symbols:
+    """Visit the annotation if any, remove the symbol from the context."""
+    yield from visit_optional(vtor, node.annotation)
+    vtor.scopes[node.arg] = None
+
+
+@SymbolsVisitor.on(arguments)
+def visit_arguments(vtor: SymbolsVisitor, node: arguments) -> Symbols:
+    """Visit the defaults values first, then the arguments names."""
+    yield from visit_sequence(vtor, node.kw_defaults)
+    yield from visit_sequence(vtor, node.defaults)
+    yield from visit_sequence(vtor, node.kwonlyargs)
+    yield from visit_sequence(vtor, node.args)
+    yield from visit_optional(vtor, node.kwarg)
+    yield from visit_optional(vtor, node.vararg)
+
+
+@SymbolsVisitor.on(comprehension)
+def visit_comprehension(vtor: SymbolsVisitor, node: comprehension) -> Symbols:
+    """Visit the iterable expression, then the target name, then the
+    predicates.
+    """
+    yield from vtor.visit(node.iter)
+    yield from vtor.visit(node.target)
+    yield from visit_sequence(vtor, node.ifs)
+
+
+@SymbolsVisitor.on(ExceptHandler)
+def visit_except_handler(vtor: SymbolsVisitor, node: ExceptHandler) -> Symbols:
+    """Visit the exception type, remove the alias from the context then
+    visit the body.
+    """
+    yield from visit_optional(vtor, node.type)
+    if node.name is not None:
+        vtor.scopes[node.name] = None
+    yield from visit_sequence(vtor, node.body)
+
+
+# STATEMENTS
+
+
+@SymbolsVisitor.on(ClassDef)
+def visit_class_def(vtor: SymbolsVisitor, node: ClassDef) -> Symbols:
+    """Visit in the following order:
+        Decorators; Base classes; Keywords; Remove name from context; Body.
+    """
+    yield from visit_sequence(vtor, node.decorator_list)
+    yield from visit_sequence(vtor, node.bases)
+    yield from visit_sequence(vtor, node.keywords)
+    vtor.scopes[node.name] = None
+    with vtor.scope():
+        yield from visit_sequence(vtor, node.body)
+
+
+@SymbolsVisitor.on(AsyncFunctionDef)
+@SymbolsVisitor.on(FunctionDef)
+def visit_function(vtor: SymbolsVisitor, node: Function) -> Symbols:
+    """Visit a function definition in the following order:
+        Decorators; Return annotation; Arguments default values;
+        Remove name from context; Arguments names; Function body.
+    """
+    yield from visit_sequence(vtor, node.decorator_list)
+    yield from visit_optional(vtor, node.returns)
+    yield from visit_sequence(vtor, node.args.kw_defaults)
+    yield from visit_sequence(vtor, node.args.defaults)
+    vtor.scopes[node.name] = None
+    with vtor.scope():
+        yield from visit_sequence(vtor, node.args.kwonlyargs)
+        yield from visit_sequence(vtor, node.args.args)
+        yield from visit_optional(vtor, node.args.kwarg)
+        yield from visit_optional(vtor, node.args.vararg)
+        yield from visit_sequence(vtor, node.body)
+
+
+@SymbolsVisitor.on(Import)
+def visit_import(vtor: SymbolsVisitor, node: Import) -> Symbols:
+    """Add the module to the current context."""
+    for alias in node.names:
+        vtor.scopes[alias.asname or alias.name] = alias.name
+        yield (alias.name, node)
+
+
+@SymbolsVisitor.on(ImportFrom)
+def visit_import_from(vtor: SymbolsVisitor, node: ImportFrom) -> Symbols:
+    """Add the symbols to the current context."""
+    for alias in node.names:
+        module = node.module or ""
+        qualified = f"{module}.{alias.name}"
+        vtor.scopes[alias.asname or alias.name] = qualified
+        yield (qualified, node)
+
+
+# EXPRESSIONS
+
+
+@SymbolsVisitor.on(Attribute)
+def visit_attribute(vtor: SymbolsVisitor, node: Attribute) -> Symbols:
+    """Postfix the seen symbols."""
+    for lhs, _ in vtor.visit(node.value):
+        yield (f"{lhs}.{node.attr}", node)
+
+
+@SymbolsVisitor.on(DictComp)
+def visit_dict_comp(vtor: SymbolsVisitor, node: DictComp) -> Symbols:
+    """Same as visit_unary_comp, except here we have a key and a value."""
+    with vtor.scope():
+        yield from visit_sequence(vtor, node.generators)
+        yield from vtor.visit(node.key)
+        yield from vtor.visit(node.value)
+
+
+@SymbolsVisitor.on(Lambda)
+def visit_lambda(vtor: SymbolsVisitor, node: Lambda) -> Symbols:
+    """Visit the arguments first, then the body."""
+    with vtor.scope():
+        yield from vtor.visit(node.args)
+        yield from vtor.visit(node.body)
+
+
+@SymbolsVisitor.on(Name)
+def visit_name(vtor: SymbolsVisitor, node: Name) -> Symbols:
+    """If the symbol is getting overwritten, then delete it from the context,
+    else yield it if it's known in this context.
+    """
+    if isinstance(node.ctx, (Del, Param, Store)):
+        vtor.scopes[node.id] = None
+    name = vtor.scopes.get(node.id)
+    if name is not None:
+        yield (name, node)
+
+
+@SymbolsVisitor.on(GeneratorExp)
+@SymbolsVisitor.on(ListComp)
+@SymbolsVisitor.on(SetComp)
+def visit_unary_comp(vtor: SymbolsVisitor, node: UnaryComp) -> Symbols:
+    """Visit the generators expressions, then the left element, the whole being
+    wrapped into a new context.
+    """
+    with vtor.scope():
+        yield from visit_sequence(vtor, node.generators)
+        yield from vtor.visit(node.elt)
diff --git a/flake8_alfred/visitor.py b/flake8_alfred/visitor.py
new file mode 100644
index 0000000..7cde3bf
--- /dev/null
+++ b/flake8_alfred/visitor.py
@@ -0,0 +1,70 @@
+"""Generic implementation of the Visitor and Dispatcher patterns."""
+
+from collections import ChainMap
+
+from typing import (
+    Any, Callable, ChainMap as ChainMapT, Dict, Generic,
+    GenericMeta, Hashable, Tuple, Type, TypeVar
+)
+
+
+A = TypeVar("A")
+B = TypeVar("B")
+C = TypeVar("C")
+
+
+class RegisterMeta(type):
+    """Register meta class. Classes that are implemented using this metaclass
+    have a `shared_dict` property visible to their subclasses, that's a
+    mapping of arbitrary keys and values.
+    """
+    @classmethod
+    def __prepare__(mcs, name: str, bases: Tuple[Type], **kwargs: Any) -> Dict:
+        dicts = (base.shared_dict for base in bases if isinstance(base, mcs))
+        return {"_shared_dict": ChainMap(*dicts).new_child()}
+
+    @property
+    def shared_dict(cls) -> ChainMapT:
+        """Returns the class shared dict."""
+        return cls._shared_dict  # type: ignore
+
+
+class GenericRegisterMeta(RegisterMeta, GenericMeta):
+    """Generic-compatible version of RegisterMeta."""
+
+
+class Dispatcher(Generic[A, B], metaclass=GenericRegisterMeta):
+    """Dispatcher base class."""
+    def dispatch(self, key: A) -> B:
+        """Returns the item associated with `key` or raise `KeyError`."""
+        return type(self).shared_dict[key]
+
+    @classmethod
+    def on(cls, key: Hashable) -> Callable[[C], C]:
+        """Register a value into the `shared_dict` class attribute."""
+        def _wrapper(value: C) -> C:
+            cls.shared_dict[key] = value
+            return value
+        return _wrapper
+
+
+class Visitor(Dispatcher[Type[A], Callable[["Visitor[A, B]", A], B]]):
+    """Visitor base class."""
+    def generic_visit(self, node: A) -> B:
+        """Generic visitor for nodes of unknown type. The default
+        implementation raises a TypeError.
+        """
+        raise TypeError(f"{type(node)} is not registered by {self}")
+
+    def visit(self, node: A) -> B:
+        """Visits a node by calling the registered function for this type of
+        nodes.
+        """
+        for base in type(node).mro():
+            try:
+                function = self.dispatch(base)
+            except KeyError:
+                pass
+            else:
+                return function(self, node)
+        return self.generic_visit(node)