This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book addresses the question of how to measure the amount of information that can be held in mind successfully at one time, or working memory capacity. It serves as an interesting historical backdrop for a broader debate about psychological theories. The book investigates working memory limits broadly defined, with reference to terminology commonly used within the field of working memory. It then describes investigations of capacity limits in preparation for the present theoretical approach to capacity. In experiential terms the book considers what is the limit on how much can be experienced at once or on how much we can be conscious of at once. Finally the chapterconcerns narrower component with which the amount that an individual can hold in mind at one time, which to be synonymous with the individual's focus of attention.