Biological macromolecules are highly concentrated in biological environments giving rise to crowding effects that impact structure, dynamics, and ultimately function. Weakly attractive non-specific interactions are amplified under such conditions leading to the dynamic formation of clusters. Recent results from molecular dynamics computer simulations of cellular systems at different scales are presented that analyze how biomolecular interactions are related to cluster formation, how cluster formation determines diffusive behavior, and under which conditions phase separated states can coexist. The computational models span from fully atomistic systems to coarse-grained models in order to cover a wide range of scales and results from simulation are compared to experiment where possible.