The operating principles of diamond-anvil high pressure cells are reviewed, with particular attention to the implications for design and construction. The diamond culets and gasket generate the pressure, and their behaviour dictates the requirements for the rest of the cell. The axial alignment mechanism is crucial, while tilt alignment is less important. The implication for piston-cylinder designs is that the clearance of the piston in the bore is critical, while the length of the piston is not. Good practice in the design of drive mechanisms is discussed. Finally, we consider alternatives to the standard piston-cylinder mechanism. Flexure movements, and their basic design rules are presented.