Plasma crystal is the term used to describe the recently discovered ordered state that a colloidal plasma may assume under certain conditions. This state resembles metals, to some extent, with the "atoms" represented by the highly negatively charged and highly ordered colloidal particles and the "electrons" by the mobile plasma ions and electrons-perhaps "heavy metal" might be an appropriate description. Plasma crystals are formed in a colloidal plasma if two conditions are met : (1) The Coulomb coupling parameter (the ratio of the Coulomb energy between neighboring particles to their kinetic energy) exceeds a certain threshold and (2) the lattice parameter (the ratio of the particle separation to the Debye length) is smaller than unity. These conditions are easy to generate in rf discharge plasmas and plasma crystallization then proceeds spontaneously. Plasma crystals have some unique properties, which make them exciting systems to study. (1) In their own right, as a hitherto unknown form of condensed plasma, they may provide many insights into basic plasma physical processes and transport effects. (2) As model systems for the detailed investigation of phase transitions, lattice defects, annealing, doping, etc., they may provide new information for a better understanding of solid state physics. (3) As test systems they may be useful for investigating nonlinear effects in "nanocrystals" (crystals with less than approximately 100 lattice planes). The unique properties, which enable these investigations and possibly many more, are (1) global charge neutrality, (2) very fast response, (3) very little damping, and (4) easy experimental control and diagnostics, i.e., detailed imaging and high temporal resolution of the dynamics of individual particles ("atoms"). We present here an overview of recent developments in this new research field.