1. Introduction 2. Defining the Terms 2.1 First Language acquisition 2.2 Second language acquisition 3. Acquisition Process 3.1 First language acquisition 3.1.1 Input 3.1.2 Production 3.1.3 Errors and Error Correction 3.2 Second Language Acquisition 3.2.1 Input 3.2.2 Production 3.2.3 Errors and Error Correction 4. Critical factors in language learning 4.1 First language Acquisition 4.1.1 External 4.1.2 Internal 4.1.2.1 Universal grammar 4.1.2.2 Mental Development 4.2 Second language Acquisition 4.2.1 External 4.2.2 Internal 4.2.2.1 Age 4.2.2.2 L1 Influence 4.2.2.3 Motivational Forces 5. Conclusion Bibliography