We first summarize work that has been done on the effects of binaries on
theoretical population synthesis of stars and stellar phenomena. Next, we
highlight the influence of stellar dynamics in young clusters by discussing a
few candidate UFOs (unconventionally formed objects) like intermediate mass
black holes, Eta Carinae, Zeta Puppis, Gamma Velorum and WR 140.Comment: Contributed paper IAU 250: Massive Stars as Cosmic Engine