In looking ahead to possible new attempts to search for extant life on Mars, the history of the Viking biological investigations is reviewed here. Scientific considerations that led to the selection of specific experimental approaches for life detection are discussed, as well as the overall results obtained from that mission. Despite extensive preflight testing of the concepts that were to be used, unanticipated artefacts arose in the actual mission. These almost certainly reflect the fact that, at that time, there were many gaps in our understanding of the physical and chemical characteristics of the Martian environment. After Viking, many of these issues still remain unresolved, and future attempts to search for extant biology should be restrained until adequate new information about potential habitable microenvironments is obtained.