SRON, Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands
Stars comprising the OB and Wolf-Rayet spectral classes, with surface
temperatures ranging from 
104-105 K, emit the bulk of their
radiation at ultraviolet wavelengths. Yet the infrared data gathered on these
objects by the ISO instruments have provided unique and powerful diagnostics
of the structure, physical conditions, and chemistry in the winds, and in the
circumstellar gas and dust environments. In addition to providing the
best view of continuum and line formation in ouflowing envelopes
in transition from accelerating to constant and rapid flow, and in various
symmetries, hot star observations with ISO have burgeoned into a new branch
of circumstellar physics in the solid state. The evolutionary scope of hot
star minerology contributes new lines of evidence for that the paths that both
high- and low-mass stars may have taken on leaving the Main Sequence.
In this paper, we review contributions brought by ISO to studies of the
early-type stars, highlighting the most significant changes brought about
in the physical, geometric, and evolutionary perspective of (1) massive stars
in the OB, Be/B[e], Luminous Blue Variable, Wolf-Rayet classes, and
(2) low-mass [WC] central stars of planetary nebulae.