In the common sense, migration is considered by many authors as a mechanism for avoiding unfavorable environments by moving to expectedly more auspicious locations at different times. In other terms, migration may represent the seasonal movement of organisms from place to place owing to the change in the environmental conditions. Consequently, two important questions arise to mind, why do some animals migrate? How would migration affect the gene pool? Whatever are the answers, the cost of migration is the power required to move to a different location and the high possibility of death as a result of this movement. The editor presents a suitable collection of topics, to achieve the goal of this book, which is explaining the migration of organisms through many examples of different groups of marine and non-marine organisms, ranging from micro-invertebrates to large vertebrates (mammals), and focusing on several aspects that are not collected together.