  P Cygni is a prototype for understanding mass loss from massive stars. This
textbook star is known first of all because of two great eruptions in the 17th
century. In the first half of this century it has given its name to a class of
stars which are characterized by spectral lines consisting of nearly
undisplaced emissions accompanied by a blue-displaced absorption component.
This characteristic P Cygni-type profile betrays the presence of a stellar
wind, but P Cygni's wind is quite unlike that of other hot supergiants. P Cygni
was the first star that showed the effects of stellar evoluton from a study of
its photometric history. It shares some common properties with the so-called
Luminous Blue Variables. However, P Cygni is a unique object. This review deals
with P Cygni's photometric properties, its circumstellar environment -
including infrared and radio observations - and its optical and ultraviolet
spectrum. Smaller sections deal with P Cygni's wind structure and evolution.
