diff --git a/lib/include/repl.h b/lib/include/repl.h
new file mode 100644
index 0000000..25368f6
--- /dev/null
+++ b/lib/include/repl.h
@@ -0,0 +1,35 @@
+#ifndef REPL_H
+#define REPL_H
+
+#include "memory_pool.h"
+
+#define REPL_LINE_BUFFER_SIZE 128
+#define REPL_RESULT_BUFFER_SIZE 128
+
+typedef enum {
+	REPL_OK,
+	REPL_ERROR,
+	REPL_EXIT,
+} repl_status_t;
+
+typedef int (*get_byte_fn_t)(void);
+typedef const expression_t *(*read_fn_t)(
+    memory_pool_t *pool, const char *input, int len);
+typedef int (*evaluate_fn_t)(const expression_t *expression);
+typedef void (*print_fn_t)(const char *output, int len);
+
+typedef struct {
+	get_byte_fn_t get_byte;
+	read_fn_t read;
+	evaluate_fn_t evaluate;
+	print_fn_t print;
+	memory_pool_t pool;
+	char line_buffer[REPL_LINE_BUFFER_SIZE];
+	char result_buffer[REPL_RESULT_BUFFER_SIZE];
+} repl_t;
+
+void init_repl(repl_t *repl);
+void step_repl(repl_t *repl);
+void run_repl(repl_t *repl);
+
+#endif
diff --git a/lib/repl.c b/lib/repl.c
new file mode 100644
index 0000000..35dd60a
--- /dev/null
+++ b/lib/repl.c
@@ -0,0 +1,30 @@
+#include "repl.h"
+
+#include <stdio.h>
+
+void init_repl(repl_t *repl)
+{
+	init_memory_pool(&repl->pool);
+}
+
+void step_repl(repl_t *repl)
+{
+	int len;
+	for (len = 0; len < REPL_LINE_BUFFER_SIZE; ++len) {
+		const int byte = repl->get_byte();
+		if ('\n' == byte)
+			break;
+		repl->line_buffer[len] = (char)byte;
+	}
+	const expression_t *e = repl->read(&repl->pool, repl->line_buffer, len);
+	const int result = repl->evaluate(e);
+	const int result_len = snprintf(
+	    repl->result_buffer, REPL_RESULT_BUFFER_SIZE, "%d\n", result);
+	repl->print(repl->result_buffer, result_len);
+}
+
+void run_repl(repl_t *repl)
+{
+	while (1)
+		step_repl(repl);
+}
diff --git a/scripts/build.sh b/scripts/build.sh
index 83f6da8..eabd2a6 100644
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -10,7 +10,9 @@ mkdir -p build
 clang $CFLAGS -c -o build/evaluator.o lib/evaluator.c
 clang $CFLAGS -c -o build/memory_pool.o lib/memory_pool.c
 clang $CFLAGS -c -o build/reader.o lib/reader.c
-ar -crs build/lib.a build/evaluator.o build/memory_pool.o build/reader.o
+clang $CFLAGS -c -o build/repl.o lib/repl.c
+ar -crs build/lib.a \
+   build/evaluator.o build/memory_pool.o build/reader.o build/repl.o
 
 # Build tests
 clang $CFLAGS -Itests -c -o build/testing.o tests/testing.c
@@ -20,6 +22,9 @@ clang $CFLAGS -o build/evaluator_tests \
 clang $CFLAGS -Itests -c -o build/reader_tests.o tests/reader_tests.c
 clang $CFLAGS -o build/reader_tests \
 	  build/reader_tests.o build/lib.a build/testing.o
+clang $CFLAGS -Itests -c -o build/repl_tests.o tests/repl_tests.c
+clang $CFLAGS -o build/repl_tests \
+	  build/repl_tests.o build/lib.a build/testing.o
 
 # Build application
 clang $CFLAGS -c -o build/main.o app/main.c
diff --git a/scripts/test.sh b/scripts/test.sh
index 5ebae56..55fa04b 100644
--- a/scripts/test.sh
+++ b/scripts/test.sh
@@ -4,5 +4,6 @@ fails=0
 
 build/evaluator_tests || fails=`expr $fails + 1`
 build/reader_tests || fails=`expr $fails + 1`
+build/repl_tests || fails=`expr $fails + 1`
 
 if [ $fails -eq 0 ]; then echo Tests OK; fi
diff --git a/tests/repl_tests.c b/tests/repl_tests.c
new file mode 100644
index 0000000..515451e
--- /dev/null
+++ b/tests/repl_tests.c
@@ -0,0 +1,107 @@
+#include "repl.h"
+#include "testing.h"
+
+#include <stdio.h>
+#include <string.h>
+
+static const char *input;
+static int input_len;
+
+static bool read_called;
+static const char *read_input;
+static int read_len;
+static const expression_t *read_result;
+
+static bool evaluate_called;
+static const expression_t *evaluate_expression;
+static int evaluate_result;
+
+static bool print_called;
+static const char *print_output;
+static int print_len;
+
+static int test_get_byte(void)
+{
+	if (0 == input_len)
+		return EOF;
+	--input_len;
+	return *input++;
+}
+
+static const expression_t *
+test_read(memory_pool_t *pool, const char *input, int len)
+{
+	(void)pool;
+	read_called = true;
+	read_input = input;
+	read_len = len;
+	return read_result;
+}
+
+static int test_evaluate(const expression_t *expression)
+{
+	evaluate_called = true;
+	evaluate_expression = expression;
+	return evaluate_result;
+}
+
+static void test_print(const char *output, int len)
+{
+	print_called = true;
+	print_output = output;
+	print_len = len;
+}
+
+static repl_t repl = {
+	.get_byte = test_get_byte,
+	.read = test_read,
+	.evaluate = test_evaluate,
+	.print = test_print,
+};
+
+static void set_up_valid_state(void)
+{
+	static const expression_t expression
+	    = { .is_number = true, .number = 1234 };
+	input = "foobar\nbarquux";
+	input_len = 14;
+	read_result = &expression;
+	evaluate_result = 4321;
+	init_repl(&repl);
+}
+
+static void read_is_called_on_first_line_line_in_input(void)
+{
+	set_up_valid_state();
+	step_repl(&repl);
+	ASSERT_TRUE(read_called);
+	ASSERT_EQUAL(6, read_len);
+	ASSERT_MEM_EQUAL("foobar", read_input, 6);
+}
+
+static void read_result_is_passed_to_evaluate(void)
+{
+	set_up_valid_state();
+	step_repl(&repl);
+	ASSERT_TRUE(evaluate_called);
+	ASSERT_TRUE(evaluate_expression->is_number);
+	ASSERT_EQUAL(1234, evaluate_expression->number);
+}
+
+static void result_of_evaluation_is_printed_with_a_newline(void)
+{
+	set_up_valid_state();
+	step_repl(&repl);
+	ASSERT_TRUE(print_called);
+	ASSERT_EQUAL(5, print_len);
+	ASSERT_MEM_EQUAL("4321\n", print_output, 5);
+}
+
+int main(void)
+{
+	TESTING_BEGIN();
+	RUN_TEST(read_is_called_on_first_line_line_in_input);
+	RUN_TEST(read_result_is_passed_to_evaluate);
+	RUN_TEST(result_of_evaluation_is_printed_with_a_newline);
+	TESTING_END();
+}