What’s a temporal relation? What’s a temporal relvar? And what’s a temporal database? This chapter distinguishes between relational and nonrelational approaches to the temporal database issue. It discusses the fundamental notion of a timestamped proposition and the fundamental significance of the propositions on, since, and during. It also briefly explains the distinction between “valid time” and “transaction time.” Importantly, it introduces the fundamental notion of an interval as a finite sequence of points, and it discusses the related notion of granularity. Finally, it revisits The Closed World Assumption and stresses the need to be extremely careful in stating predicates when timestamps are involved.