  The young star clusters we observe today are the building blocks of a new
generation of stars and planets in our Galaxy and beyond. Despite their
fundamental role we still lack knowledge about the conditions under which star
clusters form and the impact of these often harsh environments on the evolution
of their stellar and substellar members. We demonstrate the vital role
numerical simulations play to uncover both key issues. Using dynamical models
of different star cluster environments we show the variety of effects stellar
interactions potentially have. Moreover, our significantly improved measure of
mass segregation reveals that it can occur rapidly even for star clusters
without substructure. This finding is a critical step to resolve the
controversial debate on mass segregation in young star clusters and provides
strong constraints on their initial conditions.
