This presentation paper introduces the reader to a variety of representative studies in the field of second language acquisition (SLA). Focus in this special issue is on the diversity of theoretical approaches and on the specific methodologies in L2 data collection they require. An overview of the evolution of the field of SLA and related theoretical controversies is followed by a brief account of the questions addressed such as the role of input, L3 acquisition, age factors in L2 acquisition by children, and a short presentation of the different papers in this volume.