This book comprises eleven contributions dealing with various aspects of age and the age factor in the acquisition and teaching of second languages, particularly English and Spanish. The articles chosen for this monograph have been written by expert researchers in the field from Spain, Great Britain and the USA. The advances put forward in this book will certainly be of interest to researchers, scholars, and graduate students working on this topical issue in the fields of bilingualism, multilingualism, and second language teaching and learning. The book is divided into three parts. Part one deals with theoretical issues related to age and their pedagogical implications. Part two centers on age and L2 learning in school and family settings. Part three focuses on age and the relationship between aptitude and child/adult L2 learning.