The fundamental requirements for the satisfactory dimming of low-pressure hot-cathode and cold-cathode fluorescent lamps are discussed. D.C. operation is first considered, and the treatment is then developed to cover a.c. circuits. Lamp stability is considered from the points of view of smooth control of current, supply-voltage variation, and intermittent extinction. Lamp circuits are generalized in terms of Thévenin's theorem, and the ideas of voltage control and impedance control are introduced. Practical circuits are discussed, the difficulties of multiple-lamp control, and the means for overcoming these, being mentioned. The separate-impedance method of control is described by reference to several circuits, and methods of unifying the control are suggested.