  We present the first results of a comprehensive census of the stellar content
of the nearby OB associations based on Hipparcos positions, proper motions and
parallaxes for 12842 candidate member stars distributed over 21 fields on the
sky. We use a new method to identify moving groups in these fields (see de
Bruijne et al., these proceedings). Previously, astrometric membership in
nearly all the nearby OB associations was known only for stars with spectral
types earlier than B5. The Hipparcos measurements now allow us to identify
members down to late F. This census provides a firm basis for studies of
galactic and extragalactic star forming regions.
