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 .