diff --git a/lang/ast.c b/lang/ast.c
new file mode 100644
index 0000000..e69de29
diff --git a/lang/include/ivy/lang/ast.h b/lang/include/ivy/lang/ast.h
index e69de29..44792e2 100644
--- a/lang/include/ivy/lang/ast.h
+++ b/lang/include/ivy/lang/ast.h
@@ -0,0 +1,223 @@
+#ifndef IVY_LANG_AST_H_
+#define IVY_LANG_AST_H_
+
+#include <blue/core/queue.h>
+
+struct ivy_token;
+
+enum ivy_ast_node_type {
+	IVY_AST_NONE = 0x00,
+	IVY_AST_UNIT,
+	IVY_AST_OP,
+	IVY_AST_MSG,
+	IVY_AST_CLASS,
+	IVY_AST_MSGH,
+	IVY_AST_PROPERTY,
+	IVY_AST_LAMBDA,
+	IVY_AST_UNIT_PACKAGE,
+	IVY_AST_UNIT_IMPORT,
+	IVY_AST_INT,
+	IVY_AST_DOUBLE,
+	IVY_AST_STRING,
+	IVY_AST_FSTRING,
+	IVY_AST_ATOM,
+	IVY_AST_IDENT,
+	IVY_AST_FOR_LOOP,
+	IVY_AST_WHILE_LOOP,
+	IVY_AST_COND_GROUP,
+	IVY_AST_COND,
+	IVY_AST_TUPLE,
+	IVY_AST_DO,
+};
+
+enum ivy_ast_op {
+	IVY_OP_NONE = 0,
+	IVY_OP_ASSIGN,
+	IVY_OP_ADD,
+	IVY_OP_SUBTRACT,
+	IVY_OP_MULTIPLY,
+	IVY_OP_DIVIDE,
+	IVY_OP_LESS_THAN,
+	IVY_OP_GREATER_THAN,
+	IVY_OP_EQUAL,
+	IVY_OP_NOT_EQUAL,
+	IVY_OP_LESS_EQUAL,
+	IVY_OP_GREATER_EQUAL,
+	IVY_OP_AND,
+	IVY_OP_OR,
+	IVY_OP_IS,
+	IVY_OP_NOT,
+};
+
+enum ivy_ast_msgh_recipient_type {
+	IVY_AST_MSGH_NONE = 0,
+	IVY_AST_MSGH_OBJECT,
+	IVY_AST_MSGH_CLASS,
+};
+
+struct ivy_ast_node {
+	enum ivy_ast_node_type n_type;
+	b_queue_entry n_entry;
+};
+
+struct ivy_ast_unit_node {
+	struct ivy_ast_node n_base;
+};
+
+struct ivy_ast_op_node {
+	struct ivy_ast_node n_base;
+	enum ivy_ast_op n_op;
+	struct ivy_ast_node *n_left;	// NULL for unary operators.
+	struct ivy_ast_node *n_right;
+};
+
+struct ivy_ast_selector_node {
+	struct ivy_ast_node n_base;
+	/* NULL for keyword messages. */
+	struct ivy_token *n_msg_name;
+	/* queue of struct ivy_token; empty for unary messages. */
+	b_queue n_args;
+};
+
+struct ivy_ast_msg_node {
+	struct ivy_ast_node n_base;
+	struct ivy_ast_node *n_recipient;
+	struct ivy_ast_selector_node *n_sel;
+	/* queue of struct ivy_ast_node; empty for unary messages. */
+	b_queue n_arg;
+};
+
+struct ivy_ast_class_node {
+	struct ivy_ast_node n_base;
+
+	struct ivy_token *n_ident;
+
+	/* queue of struct ivy_ast_property_node */
+	b_queue n_properties;
+	/* queue of struct ivy_ast_msgh_node */
+	b_queue n_msg_handlers;
+};
+
+struct ivy_ast_msgh_node {
+	struct ivy_ast_node n_base;
+	enum ivy_ast_msgh_recipient_type n_recipient;
+	struct ivy_ast_selector_node *n_sel;
+	/* queue of struct ivy_ast_node; expressions to evaluate when message handler is executed. */
+	b_queue n_body;
+};
+
+struct ivy_ast_property_node {
+	struct ivy_ast_node n_base;
+	struct ivy_token *n_ident;
+	/* one of either:
+	 *   a) a lambda. the lambda is executed to get the property value; or,
+	 *   b) a constant value. the constant is returned as the property value.
+	 *   c) NULL. the property is write-only.
+	 */
+	struct ivy_ast_node *n_get;
+	/* one of either:
+	 *   a) a lambda. the lambda is executed with the provided value as a parameter to set the property value; or,
+	 *   b) NULL. the property is read-only.
+	 */
+	struct ivy_ast_node *n_set;
+};
+
+struct ivy_ast_lambda_node {
+	struct ivy_ast_node n_base;
+	/* queue of struct lex_token; contains the names of the lambda parameters. */
+	b_queue n_arg;
+	/* queue of struct ivy_ast_node; expressions to evaluate when lambda is executed. */
+	b_queue n_body;
+};
+
+struct ivy_ast_unit_package_node {
+	struct ivy_ast_node n_base;
+	/* queue of struct ivy_token; list of multi-part identifier parts. */
+	b_queue n_ident;
+};
+
+struct ivy_ast_unit_use_node {
+	struct ivy_ast_node n_base;
+	/* queue of struct ivy_token; list of multi-part identifier parts. */
+	b_queue n_ident;
+};
+
+struct ivy_ast_int_node {
+	struct ivy_ast_node n_base;
+	/* lex token of type IVY_TOK_INT. */
+	struct ivy_token *n_value;
+};
+
+struct ivy_ast_double_node {
+	struct ivy_ast_node n_base;
+	/* lex token of type IVY_TOK_DOUBLE. */
+	struct ivy_token *n_value;
+};
+
+struct ivy_ast_string_node {
+	struct ivy_ast_node n_base;
+	/* lex token of type IVY_TOK_STRING. */
+	struct ivy_token *n_value;
+};
+
+struct ivy_ast_fstring_node {
+	struct ivy_ast_node n_base;
+	/* list of struct ivy_ast_node. nodes are evaluated and combined in-order into a string. */
+	b_queue n_value;
+};
+
+struct ivy_ast_atom_node {
+	struct ivy_ast_node n_base;
+	struct ivy_token *n_content;
+};
+
+struct ivy_ast_ident_node {
+	struct ivy_ast_node n_base;
+	struct ivy_token *n_content;
+};
+
+struct ivy_ast_for_loop_node {
+	struct ivy_ast_node n_base;
+	/* the interation 'counter', could be an identifier or a tuple. */
+	struct ivy_ast_node *n_iterator;
+	/* the object being iterated over, an expression that evaluates to an object that can be iterated. */
+	struct ivy_ast_node *n_iterable;
+	/* queue of struct ivy_ast_node. expressions to evaluate as part of the loop. */
+	b_queue n_body;
+};
+
+struct ivy_ast_while_loop_node {
+	struct ivy_ast_node n_base;
+	/* the expression to check before each loop iteration. */
+	struct ivy_ast_node *n_cond;
+	/* queue of struct ivy_ast_node. expressions to evaluate as part of the loop. */
+	b_queue n_body;
+};
+
+struct ivy_ast_cond_group_node {
+	struct ivy_ast_node n_base;
+	/* queue of struct ivy_ast_cond_node. */
+	b_queue n_branches;
+};
+
+struct ivy_ast_cond_node {
+	struct ivy_ast_node n_base;
+	/* expression. must evaluate to true for the condition body to be evaluated. NULL for unconditional (else). */
+	struct ivy_ast_node *n_cond;
+	/* queue of struct ivy_ast_node. expressions to evaluate if the condition is true. */
+	b_queue n_body;
+};
+
+struct ivy_ast_tuple_node {
+	struct ivy_ast_node n_base;
+	/* queue of struct ivy_ast_node. each can be either an identifier, a constant, or a tuple. */
+	b_queue n_members;
+};
+
+struct ivy_ast_do_node {
+	struct ivy_ast_node n_base;
+	/* queue of struct ivy_ast_node. expressions to evaluate when the do node itself is evaluated. */
+	b_queue n_members;
+};
+
+#endif
\ No newline at end of file