High-pressure plasmas occupy a large parameter range of electron densities, electron temperatures, and gas temperatures. This diversity equally leads to the applicability of high pressure plasmas in many different application areas. A strong coupling between physics, chemistry, transport, and electromagnetics makes these plasmas a complex multidisciplinary topic and an exceptional challenge for diagnostics. Most nonequilibrium high-pressure plasmas are highly transient in nature and have strong species density gradients. This presents strong demands on temporal and spatial resolution of diagnostics. Nonequilibrium population distributions of species and high collisionality are strongly complicating factors for the interpretation of optical diagnostics. This complexity is further enhanced with the presence of a liquid phase. A summary of the main optical diagnostics used in high-pressure nonequilibrium plasmas is given, and their advantages and disadvantages are discussed.