This book presents scientific literature on treatment adherence and empirical research, as well as factors that influence it, both in Anglo-Saxon culture and Iberoamerican countries. This work summarises the updating of analysis and research works carried out by a group of researchers, composed by related psychologists and health professionals from eight Iberoamerican countries: Chile, Colombia, Cuba, Spain, United States of America, Mexico, Puerto Rico and Venezuela. All of them are professionally experienced in the health field in their respective countries of origin.