This volume is dedicated to Bill Rutherford, a long-time friend and colleague, whose commitment and dedication to the study of interlanguage has inspired many of us. The various papers deal with relevant issues in the study of interlanguage with the aim of addressing the wide range of theoretical and pedagogical questions that Rutherford has tried to answer throughout his career. This multifaceted character of the book is very appealing, and so is the inquisitive and provocative tone of several of the papers. I have chosen to group the contributions under five different headings, depending on the cognitive and behavioral sciences that constitute their main focus of attention.