  Massive OB stars play an important role in the evolution of molecular clouds
and star forming regions. The OB stars both photo--ionize molecular gas as well
as sweep up and compress interstellar gas through winds, ionization fronts, and
supernovae. In this contribution, we examine the effect massive stars have on
the formation of young stellar clusters. We first discuss the processes by
which OB stars destroy cluster--forming molecular cores, and hence terminate
star formation. We overview observational evidence that OB stars forming in
young stellar clusters destroy their parental cores on a timescale of 0.1 Myr,
and we discuss some of the implications of this result. We then summarize
extensive observations of the NGC 281 molecular cloud complex, and present
evidence that two types of triggered star formation are occurring in this
complex. Our goal is to underscore the impact massive stars have on cluster
formation over distances ranging from 0.1 pc to 300 pc.
