Published paperMappin Street, Sheffield, S1 3JD, UK Abstract: Model structure selection plays a key role in nonlinear system identification. The first step in nonlinear system identification is to determine which model terms should be included in the model. Once significant model terms have been determined, a model selection criterion can then be applied to select a suitable model subset. The well known orthogonal least squares type algorithms are one of the most efficient and commonly used techniques for model structure selection. However, it has been observed that the orthogonal least squares type algorithms may occasionally select incorrect model terms or yield a redundant model subset in the presence of particular noise structures or input signals. A very efficient integrated forward orthogonal search (IFOS) algorithm, which is assisted by the squared correlation and mutual information, and which incorporates a generalised cross-validation (GCV) criterion and hypothesis tests, is introduced to overcome these limitations in model structure selection.