**1. Introduction**

The reviews and papers in this Special Issue focus on the properties of the most luminous stars in nearby galaxies, those galaxies in which the brightest individual stars are resolved and can be observed. The most luminous stars are also the most massive and because of their intrinsic brightness and relatively short lifetimes they provide our first probes of the progress of stellar evolution in different environments.

The study of massive stars in other galaxies offers many advantages. Foremost of course is distance. Studies of stellar populations in nearby galaxies have the advantage that all the stars are at approximately the same distance, a distance that is relatively well known, especially in comparison with the uncertain distances of individual stars in our own galaxy. In the Milky Way, our observations are also limited to a relatively small volume by interstellar extinction, which can be high and uncertain at increasing distances. In external galaxies, extinction by dust is still a problem but the foreground extinction is well determined from maps of the interstellar "cirrus" or dust along the line of sight. Internal extinction within the galaxy can be variable and must still be corrected.

In this introductory review, I present a brief historical overview with emphasis on some of the main developments in the study of massive stars in nearby galaxies. Some of the first work on stars in other galaxies was driven by the identification of Cepheids for the extragalactic distance scale, see, for example, the comprehensive survey of NGC 2403 by Tammann & Sandage [1]. Other types of variables were also recognized including the first discussion of a class of luminous variables by Hubble & Sandage [2] in the Local Group spirals M31 and M33 that we now call Luminous Blue Variables (LBVs). This work was done on photographic plates and the magnitudes of individual stars were often measured by hand.

The study of individual stars and their placement on the HR Diagram requires several types of data: accurate multicolor photometry for spectral energy distributions (SEDs) and the measurement of interstellar extinction; infrared observations of circumstellar dust and mass loss; and, most important, spectroscopy, for classification, temperatures, luminosity indicators, emission lines, and evidence for

mass loss from line profiles. When observing stars in other galaxies, we also have to be concerned about foreground contamination: stars in the Milky Way seen projected against the distant galaxy. Photometry is not sufficient to remove these Galactic stars, especially those of intermediate temperature and late spectral type; spectra are required.

The primary galaxies discussed in this article are the Local Group spirals M31 and M33, and the Large and Small Magellanic Clouds due to their relative proximity and the number of surveys of their luminous stellar populations. Other Local Group galaxies are also included as well as results for stars in a few nearby spiral galaxies.
