diff --git a/ql/src/codeql_ruby/ast/Expr.qll b/ql/src/codeql_ruby/ast/Expr.qll
index e48f091dfd1..29ea9e12cdb 100644
--- a/ql/src/codeql_ruby/ast/Expr.qll
+++ b/ql/src/codeql_ruby/ast/Expr.qll
@@ -113,6 +113,21 @@ class SymbolLiteral extends Literal {
   final override string getAPrimaryQlClass() { result = "SymbolLiteral" }
 }
 
+/**
+ * A method name literal. For example:
+ * ```rb
+ * - method_name      # a normal name
+ * - +                # an operator
+ * - :method_name     # a symbol
+ * - :"eval_#{name}"  # a complex symbol
+ * ```
+ */
+class MethodName extends Literal {
+  final override MethodName::Range range;
+
+  final override string getAPrimaryQlClass() { result = "MethodName" }
+}
+
 /** A sequence of expressions. */
 class StmtSequence extends Expr {
   override StmtSequence::Range range;
diff --git a/ql/src/codeql_ruby/ast/Statement.qll b/ql/src/codeql_ruby/ast/Statement.qll
index e7af8a952a0..8e52135a3ba 100644
--- a/ql/src/codeql_ruby/ast/Statement.qll
+++ b/ql/src/codeql_ruby/ast/Statement.qll
@@ -59,6 +59,46 @@ class EndBlock extends StmtSequence, @end_block {
   final override string getAPrimaryQlClass() { result = "EndBlock" }
 }
 
+/**
+ * An `undef` statement. For example:
+ * ```rb
+ * - undef method_name
+ * - undef &&, :method_name
+ * - undef :"method_#{ name }"
+ * ```
+ */
+class UndefStmt extends Stmt, @undef {
+  final override UndefStmt::Range range;
+
+  /** Gets the `n`th method name to undefine. */
+  final MethodName getMethodName(int n) { result = range.getMethodName(n) }
+
+  /** Gets a method name to undefine. */
+  final MethodName getAMethodName() { result = getMethodName(_) }
+
+  final override string getAPrimaryQlClass() { result = "UndefStmt" }
+}
+
+/**
+ * An `alias` statement. For example:
+ * ```rb
+ * - alias alias_name method_name
+ * - undef foo :method_name
+ * - undef bar :"method_#{ name }"
+ * ```
+ */
+class AliasStmt extends Stmt, @alias {
+  final override AliasStmt::Range range;
+
+  /** Gets the new method name. */
+  final MethodName getNewName() { result = range.getNewName() }
+
+  /** Gets the original method name. */
+  final MethodName getOldName() { result = range.getOldName() }
+
+  final override string getAPrimaryQlClass() { result = "AliasStmt" }
+}
+
 /**
  * A statement that may return a value: `return`, `break` and `next`.
  *
diff --git a/ql/src/codeql_ruby/ast/internal/AST.qll b/ql/src/codeql_ruby/ast/internal/AST.qll
index 618aea0313a..d0a4bc556a3 100644
--- a/ql/src/codeql_ruby/ast/internal/AST.qll
+++ b/ql/src/codeql_ruby/ast/internal/AST.qll
@@ -31,8 +31,6 @@ module AstNode {
       or
       this = any(Generated::RestAssignment ra).getChild()
       or
-      this instanceof Generated::Alias
-      or
       this instanceof Generated::SymbolArray
       or
       this instanceof Generated::Interpolation
@@ -55,10 +53,6 @@ module AstNode {
       or
       this instanceof Generated::Character
       or
-      this = any(Generated::Alias a).getName()
-      or
-      this = any(Generated::Alias a).getAlias()
-      or
       this instanceof Generated::HeredocBody
       or
       this instanceof Generated::HeredocBeginning
@@ -72,10 +66,6 @@ module AstNode {
       this instanceof Generated::RescueModifier
       or
       this instanceof Generated::Subshell
-      or
-      this instanceof Generated::Undef
-      or
-      this = any(Generated::Undef u).getChild(_)
     }
 
     override string toString() { result = "AstNode" }
diff --git a/ql/src/codeql_ruby/ast/internal/Expr.qll b/ql/src/codeql_ruby/ast/internal/Expr.qll
index 9a7dfe22758..6a2f278b5f8 100644
--- a/ql/src/codeql_ruby/ast/internal/Expr.qll
+++ b/ql/src/codeql_ruby/ast/internal/Expr.qll
@@ -172,6 +172,30 @@ module SymbolLiteral {
   }
 }
 
+module MethodName {
+  class Range extends Literal::Range, @underscore_method_name {
+    final override Generated::UnderscoreMethodName generated;
+
+    Range() {
+      exists(Generated::Undef u | u.getChild(_) = generated)
+      or
+      exists(Generated::Alias a | a.getName() = generated or a.getAlias() = generated)
+    }
+
+    final override string getValueText() {
+      result = generated.(Generated::Token).getValue() or
+      result = generated.(SymbolLiteral).getValueText() or
+      result = generated.(Generated::Setter).getName().getValue() + "="
+    }
+
+    final override string toString() {
+      result = getValueText()
+      or
+      not exists(getValueText()) and result = generated.(SymbolLiteral).toString()
+    }
+  }
+}
+
 module StmtSequence {
   abstract class Range extends Expr::Range {
     abstract Stmt getStmt(int n);
diff --git a/ql/src/codeql_ruby/ast/internal/Method.qll b/ql/src/codeql_ruby/ast/internal/Method.qll
index ae3462af950..4d0bd75cb08 100644
--- a/ql/src/codeql_ruby/ast/internal/Method.qll
+++ b/ql/src/codeql_ruby/ast/internal/Method.qll
@@ -17,7 +17,6 @@ module Method {
 
     string getName() {
       result = generated.getName().(Generated::Token).getValue() or
-      result = generated.getName().(SymbolLiteral).getValueText() or
       result = generated.getName().(Generated::Setter).getName().getValue() + "="
     }
 
diff --git a/ql/src/codeql_ruby/ast/internal/Statement.qll b/ql/src/codeql_ruby/ast/internal/Statement.qll
index 5d94ddc0684..35b11030591 100644
--- a/ql/src/codeql_ruby/ast/internal/Statement.qll
+++ b/ql/src/codeql_ruby/ast/internal/Statement.qll
@@ -25,6 +25,28 @@ module EndBlock {
   }
 }
 
+module UndefStmt {
+  class Range extends Stmt::Range, @undef {
+    final override Generated::Undef generated;
+
+    final MethodName getMethodName(int n) { result = generated.getChild(n) }
+
+    final override string toString() { result = "undef ..." }
+  }
+}
+
+module AliasStmt {
+  class Range extends Stmt::Range, @alias {
+    final override Generated::Alias generated;
+
+    final MethodName getNewName() { result = generated.getAlias() }
+
+    final MethodName getOldName() { result = generated.getName() }
+
+    final override string toString() { result = "alias ..." }
+  }
+}
+
 module ReturningStmt {
   abstract class Range extends Stmt::Range {
     abstract Generated::ArgumentList getArgumentList();
diff --git a/ql/test/library-tests/controlflow/graph/Cfg.expected b/ql/test/library-tests/controlflow/graph/Cfg.expected
index 98762e036bd..aa36014772f 100644
--- a/ql/test/library-tests/controlflow/graph/Cfg.expected
+++ b/ql/test/library-tests/controlflow/graph/Cfg.expected
@@ -566,12 +566,12 @@ case.rb:
 
 cfg.rb:
 #    1| bar
-#-----|  -> Alias
+#-----|  -> alias ...
 
 #    1| bar
 #-----|  -> bar
 
-#    3| Alias
+#    3| alias ...
 #-----|  -> foo
 
 #    3| foo
@@ -1793,7 +1793,7 @@ cfg.rb:
 #-----|  -> ! ...
 
 #  165| ... = ...
-#-----|  -> Undef
+#-----|  -> undef ...
 
 #  165| x
 #-----|  -> ... = ...
@@ -1804,7 +1804,7 @@ cfg.rb:
 #  165| 42
 #-----|  -> - ...
 
-#  167| Undef
+#  167| undef ...
 #-----|  -> two_parameters
 
 #  167| two_parameters