This chapter presents a critical review of research, theory and practice on sport activity in childhood. It aims to identify youth sport activities that cause the development of expert performance at the highest levels of professional sport without the occurrence of any negative consequences, such as overuse injuries or burnout. The chapter explores the views of others that childhood sport activities should contain positive features and lead to several positive outcomes. The main rationale used to support early specialization is the positive relationship repeatedly found between amount of time spent practising a task or domain and level of competence. In this rationale, an earlier start age in a sport coupled with specialization enables a larger accumulation of time spent practising and, thus, higher levels of competence and achievement at a later date when compared to later start ages or not specializing.