  The sustenance of life depends on the high degree of organization that
prevails through different levels of living organisms, from subcellular
structures such as biomolecular complexes and organelles to tissues and organs.
The physical origin of such organization is not fully understood, and even
though it is clear that cells and organisms cannot maintain their integrity
without consuming energy, there is growing evidence that individual assembly
processes can be thermodynamically driven and occur spontaneously due to
changes in thermodynamic variables such as intermolecular interactions and
concentration. Understanding the phase separation \emph{in vivo} requires a
multidisciplinary approach, integrating the theory and physics of phase
separation with experimental and computational techniques. This paper aims at
providing a brief overview of the physics of phase separation and its
biological implications, with a particular focus on the assembly of
membraneless organelles. We discuss the underlying physical principles of phase
separation from its thermodynamics to its kinetics. We also overview the wide
range of methods utilized for experimental verification and characterization of
phase separation of membraneless organelles, as well as the utility of
molecular simulations rooted in thermodynamics and statistical physics in
understanding the governing principles of thermodynamically driven biological
self-assembly processes.
