Second Language Acquisition (SLA) refers to the process of an individual acquiring a second language (L2) after their first language (L1) is already fully acquired. It also refers to the scientific study of learning and acquiring an L2. In the field of SLA, there are several theories that explain how an L2 is learned and acquired by an individual. However, this thesis focuses on the theoretical framework of Stephen Krashen’s five hypotheses of SLA (Krashen, 1981) and explains the major implications of his research. This thesis then explores two essential factors that influence acquisition, age and motivation, and briefly discusses other factors that may also influence SLA. The findings demonstrate that age plays a crucial role in the efficacy of SLA, in which the optimal age for SLA is prior to late adolescence. Furthermore, motivation is another crucial factor influencing SLA, as high levels of intrinsic motivation and moderate levels of extrinsic motivation result in optimal performance for learning. The final section discusses some limitations and challenges in studying SLA, and provides pedagogical implications for both L2 learners and teachers.