This chapter examines past research on the role motivation plays in the success of learning a second language. We begin by providing a comprehensive overview of the key conceptual models that have applied the construct of motivation to second language acquisition, namely Gardner and Lambert’s seminal Socio-educational Model of Motivation on Second Language Acquisition. Next, we present an overview of more contemporary conceptual models, which are more inclusive and integrative in nature, and examine how different aspects of the learner and the learning situation might influence motivation and learning outcomes. Then, we turn to the operationalisation and measurement of second language motivation and present an overview of recent empirical work on integrative motivation and second language learning. In our final section, we discuss group differences in motivation and second language acquisition, with an emphasis on the influence of gender, age and culture/ethnicity.