The field plate test has been used to derive elastic stiffness and undrained strength parameters of clay soils. Generally the interpretations are based on the measurements of the plate load and settlement, though occasionally additional measurements have been made in an attempt to improve the interpretation. The interpretations of soil parameters from the test rely on simplified assumptions of the soil behaviour. In particular the soil-is often assumed to be linear elastic perfectly-plastic. The complexity of the stress and strain paths induced within the soil by a field plate load test belies the simplicity of the interpretations. This Thesis examines the use of the field plate load test as a method of deriving in-situ parameters of clay soils, using the finite element method to conduct a parametric study of the test. The results of these studies have been used to assess the validity of the parameters determined from the test and also to examine how the interpretation may be improved. The Thesis concludes that the interpretation of the plate test can be improved to yield non-linear elastic parameters of soil at small strain. In addition the results from a plate test can, theoretically, be used to predict the elastic load settlement response of full scale foundations. The test can also be used to measure consolidation and undrained strength characteristics of a clay soil appropriate to the analysis/design of full scale foundations of similar geometry and load direction.