Intrathymic T-cell differentiation involves the generation, expansion and selection of distinct T-lymphocyte subsets. While positive and negative selection have been a focal point of T-cell development, these events represent the final stages in a complicated sequence of differentiation steps. Here, Dale Godfrey and Albert Zlotnik summarize recent advances in our understanding of early T-cell development and describe five 'control points' that identify key events in this sequence.