The book is designed to be used in an introductory course for under-graduate or graduate students. The goal is to make the information contained herein available to students with a wide variety of background knowledge. The book can be used with those with a background in language and/or linguistics and those with little or no background in these areas. The book developed out of our belief that the complexities of the field can and should be brought to the attention of many students, both those who are intending to delve further into the field and those who are only curious about the pervasive phenomenon of learning a second language.