Rainstorms, short periods of rather intensive rainfall, constitute a much larger proportion of total rainfall in the tropics than in the mid-latitudes. Rainstorms strongly influence both the variability in time and the spatial distribution of tropical rainfall. The most important effects are due to the high rainfall intensity during these storms, which can result in serious damage in the form of floods, soil erosion or landslides, and the washing away of agricultural crops. However, rainstorms have largely been disregarded as a factor in regional and agricultural planning because of the lack of suitable basic data. Rainstorms in Tanzania are studied on the basis of daily rainfall totals, which are available for many stations and over sufficiently long periods. It is shown that these data are good indicators of the main characteristics of rainstorms. The seasonal distribution shows little difference from that of the mean rainfall. The frequency of occurrence, contribution to total rainfall and the mean rainfall intensity for all days with more than 25 mm precipitation are expressed in annual indices. These are used in the construction of quantitative maps, which illustrate the distribution of these features. The maps identify areas where the threat of rainstorms is greatest. As the development of rainstorms is largely controlled by local factors, and since the storms frequently move rather erratically, the exact location where they will strike cannot be predicted and the maps are therefore mainly indicators of probabilities.