Abstract Over the past 25 years, an evolution has taken place in the understanding of the distribution and habitat of petroleum source rocks, allowing their occurrence to be accurately predicted, their effectiveness to be modeled, and the resulting petroleum accumulations to be explained. Simultaneous developments in analytical methodology have established gas and molecular organic geochemistry as some of the most effective tools to characterize petroleum in terms of source rock characteristics, thermal evolution, migration pathways, and overall geologic history. The petroleum system concept requires a progressive shift in emphasis from rock to the fluid system to petroleum discoveries. It also defines the relationship between geologic elements and processes in time and space in order to understand the driving forces behind the distribution of hydrocarbon provinces. Several case histories from Brazilian sedimentary basins demonstrate the central role of new and emerging geochemical techniques in defining petroleum systems and identifying future exploration opportunities. Geochemistry is a powerful tool that enables the exploration geologist to target the most prospective oil and gas plays.