  We review the properties of massive star evolution in different environments,
where the major environmental factor is metallicity. Comparisons between
evolutionary models and observations of massive OB, WR stars and related
objects are presented. We also review several observations asking for future
improvements of stellar models and theoretical developments in this respect. We
summarize evolutionary scenarios for the most massive stars and try to clarify
recent questions regarding their evolutionary status as core-H or core-He
burning objects. Another environmental effect, which might affect stellar
evolution is a cluster environment with a high stellar density. As test cases
of massive star evolution in dense clusters we summarize recent work on the
densest known resolved young clusters: R136, NGC 3603, and the three Galactic
Center star clusters (the central cluster, Quintuplet and "Arches" cluster).
For the central cluster we present new comparisons between stellar parameters
of emission line stars derived by Najarro et al. (1994, 1997), and appropriate
evolutionary models. From their parameters we argue that most of these stars
can be regarded as WNL stars, and do hence not necessarily represent a peculiar
class. We suggest that some apparent differences with well known WR stars can
be understood in terms of their core burning stage and/or other changes due to
a high metallicity. Based on our present knowledge we conclude that in young
clusters with central stellar densities up to rho_c ~ 10.**(5-6) Msun/pc**3 no
compelling evidence for a secondary effect influencing the evolution of massive
stars has yet been found.
