  In this paper we review the observational data for OB stars in the SMC
concentrating on those aspects of their spectra which provide information on
processes which may strongly influence their evolution, namely mass-loss,
rotational mixing and mass-transfer. We illustrate the very weak winds now
thought to pertain to late O-dwarfs in the SMC, using HST/STIS observations of
the main sequence in the very young cluster NGC346, briefly discussing the
quantitative results for these stars, and the difficulties involved in their
determination. We show how stars with similar luminosities can have different
luminosity classes while stars with similar spectral types and luminosity
classes can have significantly different luminosities. While the weak winds of
the dwarfs present serious difficulties for the determination of wind terminal
velocities we show that the supergiants have terminal velocities comparable to
OB supergiants in the Milky Way, agreement with theory. We also summarize
recent work demonstrating that the temperature dependence of wind terminal
velocities does not follow the widely adopted step-like approximation. Finally
we review surface compositons of OB stars in the SMC finding that 42 out of 45
OB stars with detailed surface abundances are enriched in nitrogen by a factor
\~10 or more. While these enhancements are consistent with those produced by
models with rotational mixing the rotational velocities of the sample are
significantly lower than the values predicted by the modesls, indicating a
possible problem with the evolution of angular momentum in the models or
possibly in the efficiency of mixing.
