diff --git a/src/syntax/types.rs b/src/ast_types.rs
similarity index 100%
rename from src/syntax/types.rs
rename to src/ast_types.rs
diff --git a/src/main.rs b/src/main.rs
index df06de2..757044c 100755
--- a/src/main.rs
+++ b/src/main.rs
@@ -6,6 +6,8 @@ mod syntax;
 mod lexic;
 mod token;
 mod semantic;
+mod ast_types;
+mod symbol_table;
 
 const VERSION: &str = "0.0.1";
 
diff --git a/src/repl/mod.rs b/src/repl/mod.rs
index 7bd8c67..9d8bb74 100755
--- a/src/repl/mod.rs
+++ b/src/repl/mod.rs
@@ -1,19 +1,19 @@
 use std::io::{self, Write};
 
+use crate::symbol_table::SymbolTable;
+
 use super::lexic;
 use super::syntax;
+use super::semantic;
 
 fn compile(input: &String) {
     let _tokens = lexic::get_tokens(input);
 
     match _tokens {
         Ok(tokens) => {
-            for token in tokens {
-                print!("[{:?} {}] ", token.token_type, token.value);
-            }
-            println!("");
-
-            let _ast = syntax::construct_ast(&Vec::new());
+            let mut ast = syntax::construct_ast(&tokens).unwrap();
+            let mut table = SymbolTable::new();
+            let new_ast = semantic::check_ast(&mut ast, &mut table);
         },
         Err(error) => {
             eprintln!("Error scanning.\n{} at pos {}", error.reason, error.position)
diff --git a/src/semantic/mod.rs b/src/semantic/mod.rs
index 7f79cb4..f240336 100644
--- a/src/semantic/mod.rs
+++ b/src/semantic/mod.rs
@@ -1,4 +1,34 @@
+use crate::symbol_table;
 
-pub fn check_ast() {
-    
+use super::symbol_table::SymbolTable;
+use super::ast_types::ModuleAST;
+
+pub fn check_ast<'a>(ast: &'a mut ModuleAST, symbol_table: &'a mut SymbolTable) -> Option<ModuleAST<'a>> {
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /*
+    val identifier = 20
+
+    [Binding]
+    | identifier
+    | [Expression]
+      | [Number]
+        | 20
+
+    - Check [Expression] is valid
+    - Check type of [Expression]
+    - Check if `identifier` already exists in the symbol table
+    - Create entry in symbol table
+
+    ->
+
+    SymbolTable {
+        identifier: Num
+    }
+     */
 }
diff --git a/src/symbol_table.rs b/src/symbol_table.rs
new file mode 100644
index 0000000..2e53bd3
--- /dev/null
+++ b/src/symbol_table.rs
@@ -0,0 +1,46 @@
+use std::collections::HashMap;
+
+// Primitive datatypes
+pub const _NUMBER: &str = "Num";
+pub const _STRING: &str = "Str";
+pub const _BOOLEAN: &str = "Bool";
+
+pub struct SymbolTable {
+    table: HashMap<String, String>
+}
+
+impl SymbolTable {
+    pub fn new() -> SymbolTable {
+        let mut symbol_table = HashMap::<String, String>::new();
+
+        SymbolTable {
+            table: symbol_table,
+        }
+    }
+
+    pub fn add(&mut self, identifier: &str, datatype: &str) {
+        self.table.insert(String::from(identifier), String::from(datatype));
+    }
+
+    pub fn test(&self, identifier: &str) -> bool {
+        return self.table.contains_key::<String>(&String::from(identifier));
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn should_create() {
+        let mut _table = SymbolTable::new();
+    }
+
+    #[test]
+    fn should_add_identifier() {
+        let mut table = SymbolTable::new();
+        table.add("identifier", _NUMBER);
+        assert_eq!(true, table.test("identifier"))
+    }
+}
diff --git a/src/syntax/expression.rs b/src/syntax/expression.rs
index b28bc0d..59d14bd 100644
--- a/src/syntax/expression.rs
+++ b/src/syntax/expression.rs
@@ -1,5 +1,5 @@
 use crate::token::{Token, TokenType};
-use super::types::Expression;
+use super::ast_types::Expression;
 
 
 pub fn try_parse(tokens: &Vec<Token>, pos: usize) -> Option<Expression> {
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index ed3bfb2..66e1717 100755
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -2,9 +2,9 @@ use super::token::Token;
 
 mod expression;
 mod val_binding;
-mod types;
+use super::ast_types;
 
-use types::ModuleAST;
+use ast_types::ModuleAST;
 
 /// Constructs the Misti AST from a vector of tokens
 pub fn construct_ast<'a>(tokens: &'a Vec<Token>) -> Result<ModuleAST<'a>, String> {
diff --git a/src/syntax/val_binding.rs b/src/syntax/val_binding.rs
index f91a3cf..dc50712 100644
--- a/src/syntax/val_binding.rs
+++ b/src/syntax/val_binding.rs
@@ -1,5 +1,5 @@
 use crate::token::{Token, TokenType};
-use super::types::{Binding, ValBinding};
+use super::ast_types::{Binding, ValBinding};
 use super::expression;
 
 // Should return a 3 state value: