The Three Functional States of Proteins explores how structured proteins, intrinsically disordered proteins, and phase separated proteins contribute to the complexity of cellular life, and offers insights into their roles in both health and disease. It discusses the latest research findings and highlight groundbreaking discoveries and innovative methodologies used to study these protein states. Traditionally, the different states of proteins have been defined based on their structures and functions. However, it is becoming increasingly clear that these criteria alone may not be sufficient to capture the complex and multifaceted properties of these molecules. Definitions based on thermodynamics and kinetics are now recognized as potentially more appropriate for comprehensively understanding protein states. Emerging evidence indicates that under physiological conditions, a majority of proteins possess the capability to exist in and transition between the native, droplet, and amyloid states. These distinct states play crucial roles in various cellular functions, influenced significantly by their physicochemical and structural properties. The book also considers the interactions among these states and discusses how their internal organization as individual molecules, as well as their collective organization as molecular assemblies are stabilized. Furthermore, it examines the processes by which these states are formed and the cellular functions associated with each specific state.