Liquid-liquid phase separation (LLPS) has recently emerged as a pervasive and fundamental strategy for organizing cellular contents and regulating biological processes. Indeed, LLPS has been revealed to regulate a staggering and rapidly expanding array of cellular activities, including aspects of DNA replication, transcription, DNA repair, RNA metabolism, receptor signaling, synapse formation, and more. In cells, LLPS leads to the assembly of biomolecular condensates that span a vast range of sizes and complexities, from nanometer-scale structures composed of a few peptide chains, such as the nucleosome core, to micron-scale structures composed of thousands of biomolecules, such as the pre- and post-synaptic compartments of neurons. In addition, it is increasingly appreciated that disturbance of biological phase transitions is a primary driver of disease, most notably in neurodegeneration and cancer.