Understanding the process of second language learning is the most crucial foundation for all practices relevant to language teaching, curriculum design, and learning support services. This introductory course on second language acquisition (SLA) aims to explore how languages are learned in both formal and informal contexts, and how first language learning is different from second or third language learning. Key issues that underlie English language acquisition, with special emphasis on those that are pertinent to English language teachers are examined. Affective factors contributing to the development, maintenance, and attrition of languages are analyzed and discussed to inform educational practices of how to best facilitate the language learning process. They include age, individual differences, aptitude, cognition, affect, motivation, and social dimensions of learning.