  We have developed a new procedure to identify moving groups, and have applied
it to carry out a census of the nearby OB associations based on Hipparcos
parallaxes and proper motions (see de Bruijne et al. and de Zeeuw et al.
elsewhere in this volume). Here we present three illustrative cases. For
$\alpha$ Persei our method allows us to refine the bright end of the membership
list, while for Collinder 121 the Hipparcos data change the whole appearance of
the association. Finally, we report the discovery of a new association in the
field of Cepheus OB2.
