This volume contains papers presented at the Workshop on Branching Processes and Their Applications (WBPA09), held in Badajoz, Spain, April 20-23, 2009. These papers deal with theoretical and practical aspects of branching process theory, showing it to be an area of active and interesting research. They clearly indicate the vitality of the theoretical research on this topic, as well as the importance of branching concepts in the development of theoretical approaches to solving new problems in applied fields such as Epidemiology, Cell Kinetics, Genetics, and, of course, Population Dynamics. The topics covered by the papers have been classified into the following areas: Population Growth Models in Random and Varying Environments, Special Branching Processes, Limit Theorems and Statistics, Applications in Cell Kinetics and Genetics, Applications in Epidemiology, Two-Sex Branching Models. This book represents a valuable reference on the contemporary branching process theory, presenting also many open problems which will lead to future research.