Combinativity is one of the most characteristic features of human thinking: this ability enables us to create new constructions and various compositions from certain elements. A number of psychological trends deal with the focal system of operations of this ability, its functions have been best elaborated in the Piagetian theory. This book presents results of research with the aim of a detailed analysis of the system of combinative operations, describing its inner structure and its developmental process. After a summary of the results of former studies, the general conception of combinative ability is outlined and combinative operational ability as part of it is defined. The results of an assessment involving hundreds of test tasks (carried out in three age groups), are utilized to analyze the manifestations of combinative operations in different contents, the structure of system of operations, and the factors that determine the functioning and development of the operations. The measuring instruments allow a better assessment of the development of combinative operations, and the results of the study provide a foundation for the effective improvement of the operational abilities of thinking.