TRMM products are used to document the three dimensional structure of precipitation systems at various phases of the West African Monsoon. There is a remarkable contrast between the continental systems and those of the ITCZ, which are aligned along the Guinean coast during the pre-monsoon period. For example, the continental pre-monsoon systems are deep, intense and scattered, while those along the coast are relatively shallow, moderately intense and wide-spread. The onset of the monsoon involves the development of systems of the latter kind in the continental interior. The associated evolution of latent heating profiles are compared with those from the global reanalyses and implications for the three dimensional structure of the monsoon circulations are inferred.