The structure and performance of locally optimum detectors for composite signals in additive and multiplicative noise are investigated. A generalised model representing the effects of multiplicative noise and additive noise is considered for signal detection problems. The signals considered here are composite signals which contain both deterministic and stochastic signal components. Locally optimum detectors consist of nonlinearities followed by an accumulator, whose output is then compared with a threshold. The detection nonlinearities for a probability density function are obtained to show the detection structures. Finite sample-size performance characteristics of the locally optimum detector are obtained and are compared with those of other detectors.