**1. Introduction**

Massive stars play a major role in the evolution of their host galaxies. Via stellar winds, they strongly enrich the interstellar medium (ISM) with chemically processed material and deposit large amounts of momentum and energy into their surroundings during their entire lifetime, from the main-sequence up to their final fate as spectacular supernova explosions (e.g., [1–3]). The released energy provides the ionizing radiation, substantially supplies the global energy budget of the host galaxy and significantly contributes to shaping the local ISM, whereas the released material condenses into molecules and dust, providing the cradles for the next generation of stars and planets (e.g., [4,5]).

Despite their grea<sup>t</sup> importance, stellar evolution theory is most uncertain for massive stars due to the often still poor understanding of some physical processes in the stellar interiors (e.g., core convective overshooting, chemical diffusion, internal differential rotation law and angular momentum transport), the excitation and propagation of pulsation instabilities within their atmospheres, the amount of mass loss via stellar (often asymmetric) winds and (irregular) mass ejections, and the role of binarity for certain phases.

From an observational point of view, the post-main sequence domain within the Hertzsprung–Russell (HR) diagram is populated with various types of extreme massive stars. These are found to be in transition phases, in which the stars shed huge amounts of material into their environments, typically via episodic, sometimes even eruptive events. These objects are luminous super- or hypergiants populating the upper part of the HR diagram and spreading from spectral type O to F or even later. The ejected material thereby accumulates in either nebulae, shells, or even disk-like structures.

The mass-loss of massive stars not only critically depends on the physical parameters, such as mass, effective temperature, and rotation speed, but also on the chemical composition of the star. The amount of mass that is lost, within each individual evolutionary phase, determines the fate of the object. It is thus not surprising that relative numbers of various types of massive stars can change drastically among galaxies with different metallicities (e.g., [6]). For reliable predictions of the evolutionary path of massive stars in any environment, solid constraints on the physical parameters, used in modern stellar evolution models, are indispensable. To obtain such constraints, the properties of the members within each class of objects need to be studied in grea<sup>t</sup> detail and within a variety of environments. This requires statistically significant samples of stars in each class of objects, suitable for a detailed analysis. The star-forming galaxies within the local Universe, in which the metallicities spread over a factor of about 25 between the most metal poor and the most metal rich representative, are the most ideal sites to tackle this challenge.

This review is devoted to the B[e] supergiants, which comprise one of the various classes of extreme massive stars in transition. The article is structured as follows. First, an overview on the general properties of these stars is given based mostly on the well-studied sample within the Magellanic Clouds (Section 2), followed by a review on how these objects are searched for in various environments (Section 3). A census of the currently known objects in the Local Group galaxies and slightly beyond is presented in Section 4, based on a critical inspection of the properties of the individual candidates. The discussion of the B[e]SG samples and our conclusions are finally summarized in Section 5.
