Publisher Summary This chapter discusses ionic fluids near critical points and at high temperatures. Studies on critical phenomena, however, have until recently mostly avoided the complications introduced by charged species, while traditional work on ionic fluids has avoided the complications that arise in the vicinity of the solvent's or mixture's critical point. Recently, there has been a concerted effort, both experimental and theoretical, to arrive at a better understanding of these issues. The chapter discusses some of this work and attempt to put it into perspective. From the standpoint of critical-phenomena theory, the behavior of thermodynamic and transport properties of pure fluids and their mixtures is known and understood in considerable detail. Serious questions exist; however, as to the applicability of the methodology when long-range Coulombic forces are present. These questions have recently generated substantial interest, controversy, and experimental and theoretical research, and some answers are beginning to emerge.