**Michaela Kraus**

Astronomical Institute, Czech Academy of Sciences, Friˇcova 298, 251 65 Ondˇrejov, Czech Republic; michaela.kraus@asu.cas.cz

Received: 7 August 2019; Accepted: 25 September 2019; Published: 29 September 2019

**Abstract:** Stellar evolution theory is most uncertain for massive stars. For reliable predictions of the evolution of massive stars and their final fate, solid constraints on the physical parameters, and their changes along the evolution and in different environments, are required. Massive stars evolve through a variety of short transition phases, in which they can experience large mass-loss either in the form of dense winds or via sudden eruptions. The B[e] supergiants comprise one such group of massive transition objects. They are characterized by dense, dusty disks of ye<sup>t</sup> unknown origin. In the Milky Way, identification and classification of B[e] supergiants is usually hampered by their uncertain distances, hence luminosities, and by the confusion of low-luminosity candidates with massive pre-main sequence objects. The extragalactic objects are often mistaken as quiescent or candidate luminous blue variables, with whom B[e] supergiants share a number of spectroscopic characteristics. In this review, proper criteria are provided, based on which B[e] supergiants can be unambiguously classified and separated from other high luminosity post-main sequence stars and pre-main sequence stars. Using these criteria, the B[e] supergiant samples in diverse galaxies are critically inspected, to achieve a reliable census of the current population.

**Keywords:** stars: massive; stars: emission line, Be; supergiants; stars: winds, outflows; circumstellar matter
