The bijel is a soft composite material with unusual characteristics that make it suitable, for example, for catalysis, filtration and electrode/electrolyte applications. The name is an acronym for bicontinuous interfacially jammed emulsion gel; it is a member of the family of emulsions with interfaces stabilized by colloidal particles. Conventional particle-stabilized (Pickering–Ramsden) emulsions have a dispersed liquid phase in the form of droplets and a continuous liquid phase that surrounds them. A bijel has two continuous liquid phases that are mutually entangled in a tortuous pattern, with a particle-stabilized interface between. Bijels were originally conceived in silico and conventionally fabricated by arresting the spinodal pattern of phase-separating liquids. The purpose of this chapter is to present the bijel concept as initially developed. This provides the foundation for the more recent innovations covered in subsequent chapters. We begin by putting the bijel idea in the context of the liquid-crystal research that immediately preceded it. We then explain the practicalities of making bijels, the processing route and the characteristics of the final samples. We briefly mention related research on freeze-casting porous ceramics, which occurred in parallel and is another example of using a phase transition in a host solvent to structure colloidal particles. Finally, we highlight some very recent research on carboxysomes, where self-organization driven by phase transition kinetics is being used in a very different context.