This document provides an overview of the changing field of maintenance over time. It discusses the evolution from the first generation of maintenance prior to WWII, to the second generation in the 1950s-60s focused on preventative maintenance, to the third generation since the mid-1970s with new expectations, research, and techniques. The document then discusses key maintenance concepts including functions and performance standards, objectives, failure modes and effects, consequences, age and deterioration, scheduled restoration tasks, scheduled discard tasks, and safe-life limits.