The present book comprehensively overviews the numerous hypotheses and theories of Second Language Acquisition (SLA)? where, among others, these theories deal with the implications of developmental stages of a second language, individual learner differences, internal and external factors (like sex or age), interaction between speakers and listeners, supplying the learners with adequate input, and reflecting on the learner’s output. Moreover, this monograph discusses in detail the role of the availability of input and interaction during the acquisition process (with a particular emphasis on Krashen''s and Long’s theories). The theoretical work on the relation between input viz. interaction and SLA is supported by an empirical study that underlines that the (second language) acquisition process can only be successful if input and interaction are tailored to the individual needs of the learners. Therefore, students of linguistics, people who would like to get an idea of what SLA is, as well as theoretical and empirical researchers will find this book compelling.