This is a tutorial introduction to assertional reasoning based on temporal logic. The objective is to provide a working familiarity with the technique. We use a simple system model and a simple proof system, and we keep to a minimum the treatment of issues such as soundness, completeness, compositionality, and abstraction. We model a concurrent system by a state transition system and fairness requirements. We reason about such systems using Hoare logic and a subset of linear-time temporal logic, specifically, invariant assertions and leads-to assertions. We apply the method to several examples.