diff --git a/golox/cmd/golox/main.go b/golox/cmd/golox/main.go
new file mode 100644
index 0000000..2b335a0
--- /dev/null
+++ b/golox/cmd/golox/main.go
@@ -0,0 +1,20 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/AYM1607/crafting-interpreters/golox/internal/runner"
+)
+
+func main() {
+	argc := len(os.Args)
+	if argc > 2 {
+		fmt.Println("Usage: golox [script]")
+		os.Exit(64)
+	} else if argc == 2 {
+
+	} else {
+		runner.RunPrompt()
+	}
+}
diff --git a/golox/internal/runner/errors.go b/golox/internal/runner/errors.go
new file mode 100644
index 0000000..307242a
--- /dev/null
+++ b/golox/internal/runner/errors.go
@@ -0,0 +1,18 @@
+package runner
+
+import "fmt"
+
+func emitError(line int, message string) {
+	report(line, "", message)
+}
+
+func report(line int, where, message string) {
+	fmt.Printf(
+		"[%d] Error%s: %s\n",
+		line,
+		where,
+		message,
+	)
+	// TODO: The book sets `hadError` as true here, need to figure out where
+	// that's used.
+}
diff --git a/golox/internal/runner/runner.go b/golox/internal/runner/runner.go
new file mode 100644
index 0000000..6ebc18f
--- /dev/null
+++ b/golox/internal/runner/runner.go
@@ -0,0 +1,37 @@
+package runner
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func RunPrompt() {
+	s := bufio.NewScanner(os.Stdin)
+	fmt.Print("> ")
+	for s.Scan() {
+		line := s.Text()
+		Run(line)
+		// TODO: resed hadError wherever it is set.
+		fmt.Print("> ")
+	}
+}
+
+func RunFile(path string) error {
+	fBytes, err := os.ReadFile(path)
+	if err != nil {
+		return fmt.Errorf("could not read script file: %w", err)
+	}
+	Run(string(fBytes))
+	// TODO: check hadError and exit with a 65 code if so.
+	return nil
+}
+
+func Run(source string) {
+	s := NewScanner(source)
+	tokens := s.ScanTokens()
+
+	for _, t := range tokens {
+		fmt.Println(t)
+	}
+}
diff --git a/golox/internal/runner/scanner.go b/golox/internal/runner/scanner.go
new file mode 100644
index 0000000..4d114a7
--- /dev/null
+++ b/golox/internal/runner/scanner.go
@@ -0,0 +1,80 @@
+package runner
+
+type Scanner struct {
+	source string
+
+	// State.
+	tokens  []Token
+	start   int
+	current int
+	line    int
+}
+
+func NewScanner(source string) *Scanner {
+	return &Scanner{
+		source: source,
+		tokens: []Token{},
+
+		start:   0,
+		current: 0,
+		line:    1,
+	}
+}
+
+func (s *Scanner) ScanTokens() []Token {
+	for !s.isAtEnd() {
+		s.start = s.current
+		s.scanToken()
+	}
+
+	s.tokens = append(s.tokens, NewToken(EOF, "", nil, s.line))
+	return s.tokens
+}
+
+func (s *Scanner) scanToken() {
+	c := s.advance()
+	switch c {
+	case '(':
+		s.addToken(LPAREN)
+	case ')':
+		s.addToken(RPAREN)
+	case '{':
+		s.addToken(LBRACE)
+	case '}':
+		s.addToken(RBRACE)
+	case ',':
+		s.addToken(COMMA)
+	case '.':
+		s.addToken(DOT)
+	case '-':
+		s.addToken(MINUS)
+	case '+':
+		s.addToken(PLUS)
+	case ';':
+		s.addToken(SEMI)
+	case '*':
+		s.addToken(STAR)
+	}
+}
+
+func (s *Scanner) advance() byte {
+	idx := s.current
+	s.current += 1
+	return s.source[idx]
+}
+
+func (s *Scanner) addToken(typ TokenType) {
+	s.addTokenWithLiteral(typ, nil)
+}
+
+func (s *Scanner) addTokenWithLiteral(typ TokenType, literal interface{}) {
+	lexme := s.source[s.start:s.current]
+	s.tokens = append(
+		s.tokens,
+		NewToken(typ, lexme, literal, s.line),
+	)
+}
+
+func (s *Scanner) isAtEnd() bool {
+	return s.current >= len(s.source)
+}
diff --git a/golox/internal/runner/token.go b/golox/internal/runner/token.go
new file mode 100644
index 0000000..eb37604
--- /dev/null
+++ b/golox/internal/runner/token.go
@@ -0,0 +1,33 @@
+package runner
+
+import "fmt"
+
+type Token struct {
+	Type    TokenType
+	Lexme   string
+	Literal interface{}
+	Line    int
+}
+
+func NewToken(
+	typ TokenType,
+	lexme string,
+	lit interface{},
+	line int,
+) Token {
+	return Token{
+		Type:    typ,
+		Lexme:   lexme,
+		Literal: lit,
+		Line:    line,
+	}
+}
+
+func (t Token) String() string {
+	return fmt.Sprintf(
+		"%s %s %v",
+		t.Type,
+		t.Lexme,
+		t.Literal,
+	)
+}
diff --git a/golox/internal/runner/token_type.go b/golox/internal/runner/token_type.go
new file mode 100644
index 0000000..2f4950f
--- /dev/null
+++ b/golox/internal/runner/token_type.go
@@ -0,0 +1,53 @@
+package runner
+
+type TokenType string
+
+const (
+	// Single character tokens.
+	LPAREN TokenType = "LPAREN"
+	RPAREN TokenType = "RPAREN"
+	LBRACE TokenType = "LBRACE"
+	RBRACE TokenType = "RBRACE"
+	COMMA  TokenType = "COMMA"
+	DOT    TokenType = "DOT"
+	MINUS  TokenType = "MINUS"
+	PLUS   TokenType = "PLUS"
+	SEMI   TokenType = "SEMI"
+	SLASH  TokenType = "SLASH"
+	STAR   TokenType = "STAR"
+
+	// One or two character tokens.
+	BANG        TokenType = "BANG"
+	BANG_EQUAL  TokenType = "BANG_EQUAL"
+	EQUAL       TokenType = "EQUAL"
+	EQUAL_EQUAL TokenType = "EQUAL_EQUAL"
+	GT          TokenType = "GT"
+	GTE         TokenType = "GTE"
+	LT          TokenType = "LT"
+	LTE         TokenType = "LTE"
+
+	// Literals.
+	IDENT  TokenType = "IDENT"
+	STRING TokenType = "STRING"
+	NUMBER TokenType = "NUMBER"
+
+	// Keywords
+	AND    TokenType = "AND"
+	CLASS  TokenType = "CLASS"
+	ELSE   TokenType = "ELSE"
+	FALSE  TokenType = "FALSE"
+	FUN    TokenType = "FUN"
+	FOR    TokenType = "FOR"
+	IF     TokenType = "IF"
+	NIL    TokenType = "NIL"
+	OR     TokenType = "OR"
+	PRINT  TokenType = "PRINT"
+	RETURN TokenType = "RETURN"
+	SUPER  TokenType = "SUPER"
+	THIS   TokenType = "THIS"
+	TRUE   TokenType = "TRUE"
+	VAR    TokenType = "VAR"
+	WHILE  TokenType = "WHILE"
+
+	EOF TokenType = "EOF"
+)