**1. Introduction**

The standard model of massive star evolution follows a rapid progression from the main-sequence, through a blue supergiant (BSG) phase, to the red supergiant (RSG) branch, to terminal supernova (SN) explosion. However, surveys of the brightest supergiants revealed an empirical upper luminosity limit to stars on the Hertzprung-Russell (HR) diagram [1]. This limit suggests that stars above some initial zero-age main-sequence (ZAMS) mass ( ≈30–40 M) do not evolve to the RSG branch on the HR Diagram and therefore follow an alternative evolutionary pathway. Since the 1980s, both observational and modeling studies have attempted to describe and constrain the stellar populations and instabilities at the upper luminosity boundary, as well as explore the local environments that influence these massive stars during both their main- and post-main-sequence lives.

It has long been established that massive stars at any stage of evolution provide a favorable environment for enhanced mass-loss in their stellar winds due to low surface gravity (*g*) in their outer atmospheres. The outer circumstellar (CS) material is only tenuously gravitationally bound to the star itself. Indeed, mass-loss rates for RSGs range from 10−<sup>6</sup> M yr<sup>−</sup><sup>1</sup> [2,3] to as high as 10−<sup>4</sup> M yr<sup>−</sup><sup>1</sup> in extreme supergiant stars like VY CMa [4]—mass-loss rates that represent a significant fraction of a star's initial mass being shed during its post-main-sequence lifetime. The evolution and terminal state of a massive star is ultimately governed not just by its ZAMS mass but also by these drastic changes in total stellar mass and outer envelope conditions. We refer to these changes in stellar mass through ejection of CS material as the "mass-loss history." In this chapter, we summarize some of the literature on the mass-loss histories of evolved supergiant stars and the evidence for post-red supergiant evolution both in observational studies of the circumstellar ejecta and in evolutionary models that predict the effect of various mass-loss mechanisms on massive star evolution.
