Cells compartmentalize their intracellular environment to orchestrate countless simultaneous biochemical processes. Many intracellular tasks rely on membrane-less organelles, multicomponent condensates that assemble by liquid-liquid phase separation. A decade of intensive research has provided a basic understanding of the biomolecular driving forces underlying the form and function of such organelles. Here we review the technologies enabling these developments, along with approaches to designing spatiotemporally actuated organelles based on multivalent low-affinity interactions. With these recent advances, it is now becoming possible both to modulate the properties of native condensates and to engineer entirely new structures, with the potential for widespread biomedical and biotechnological applications.