A current flurry of research has revealed that living cells contain a variety of membrane-less organelles that play critical roles in cell physiology and disease. They are formed via liquid phase condensation of highly conformationally flexible intrinsically disordered proteins (IDPs) that engage in a multitude of transient, multivalent, noncovalent interactions resulting in phase separation into mesoscopic liquid-like compartments. These liquid-like condensates can undergo maturation into pathological gel-like or solid-like aggregates in deadly human diseases. However, the fundamental molecular origin of phase separation remains elusive. I will discuss our recent results that shed lights into the unusual phase behavior of IDPs that undergo liquid-liquid phase separation leading to the formation of liquid droplets that can eventually mature into more ordered aggregates. Using a diverse array of unique and sensitive optical tools, we have been able to directly capture an intriguing symphony of molecular events that govern the phase transition. Our studies illuminate the key molecular drivers of IDP phase transition that is involved in both function and disease.