This chapter describes some aspects of research on the cooperative breeding system of the long tailed tit Aegithalos caudatus. The principal aim of the chapter is to bring together evidence from various sources concerning the role of kin selection and ecological constraints in the evolution of cooperative breeding in this species. Many studies of vertebrate cooperative breeding systems have shown that helpers assist relatives in raising their offspring. A major reason for initiating a study on long tailed tits is that their helping behavior is atypical and does not conform to this sequence of events because all helpers are failed breeders that ‘‘redirect’’ their care to become helpers. The ability to discriminate between kin and non kin plays a major role in the evolution of social behavior. The direct fitness benefits of helping are those that enhance the personal reproductive success of helpers. The direct fitness benefits of helping are those that enhance the personal reproductive success of helpers. Helpers may gain indirect fitness benefits either by helping relatives to increase productivity of their current breeding attempt, or by reducing the reproductive costs of related breeders, thereby enhancing their survival. The main conclusions regarding the role of kin selection in the evolution of cooperative breeding in long tailed tits are have been listed out: (1) helpers exhibit a kin preference in helping behavior using a learned vocal kin recognition mechanism, (2) helpers increase the productivity of their relatives by increasing recruitment of fledglings of the helped brood, and (3) the kin selected fitness benefit of helping is the sole source of inclusive fitness for a substantial proportion of individuals.