The past 50 years have witnessed achievement of a set of widely attested empirical findings on major research issues in SLA. They pertain to such matters as cross-linguistic influence; processes and sequences in interlanguage development; age effects; incidental and intentional, learning and implicit and explicit knowledge; the role of the linguistic environment and of the broader social context as sources of positive and negative evidence and of opportunities for input, interaction and output; and effects of individual differences in language aptitudes and other cognitive and affect variables. Robust findings in any scientific field constitute empirical 'problems' that require explanation and motivate theoretical work. In this inaugural volume of Brill Research Perspectives in Multilingualism and Second Language Acquisition, Michael Long, Gisela Granena and Yucel Yilmaz review work on a selection of these issues, and note implications of some of the work for language teaching, educational language planning, human migration, and other important matters of social concern.