**Roberta M. Humphreys**

Minnesota Institute for Astrophysics, School of Physics and Astronomy, 116 Church St. SE, University of Minnesota, Minneapolis, MN 55455, USA; roberta@umn.edu

Received: 24 July 2019; Accepted: 14 August 2019; Published: 23 August 2019

**Abstract:** Several decades of observations of the most massive and most luminous stars have revealed a complex upper HR Diagram, shaped by mass loss, and inhabited by a variety of evolved stars exhibiting the consequences of their mass loss histories. This introductory review presents a brief historical overview of the HR Diagram for massive stars, highlighting some of the primary discoveries and results from their observation in nearby galaxies. The sections in this volume include reviews of our current understanding of different groups of evolved massive stars, all losing mass and in different stages of their evolution: the Luminous Blue Variables (LBVs), B[e] supergiants, the warm hypergiants, Wolf–Rayet stars, and the population of OB stars and supergiants in the Magellanic Clouds.

**Keywords:** massive stars; local group; supergiants; Magellanic Clouds; M31; M33
