A retrospective study is a study that looks backwards in time. For example, we find people that are already dead and try to figure out why they died. A retrospective study is fast. Since the subjects are already dead; we just have to tabulate all the results. The one problem is that it's hard to interview a dead person. In contrast, a prospective study looks forward in time. For example, we select a group of subjects and sit around and watch them for a decade.