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This paper discusses characteristics of a class of devices that use a saturable absorber irradiated by an optical control pulse, or gating pulse, to alter the shape of a transmitted optical signal. We consider two device geometries: a longitudinally gated cell, in which signal and gate beams are nearly parallel, and a transverse cell, in which the medium is bleached by a gating pulse applied at right angles to the signal beam. The longitudinal device permits synthesis of rising exponential signal pulses with arbitrary, subnanosecond time constants. The transverse geometry can provide pulses of complex shape and permits independent optimization of several device parameters. The dynamics of the saturable absorber (with particular emphasis on liquid dye systems) is analyzed in detail, and a pulse-shape synthesis procedure is outlined. Spurious nonlinear effects are considered, and an example of pulse shaping for laser-driven fusion is given.