The problem of cavity expansion in unsaturated soils is investigated. A unified constitutive model formulated in a critical state framework using the concepts of effective stress and bounding surface plasticity theory is adopted. Consideration is given to the effects of suction and particle crushing in the definition of the critical state and the evolution of the bounding surface. The model accurately captures stress-strain behaviour for a range of load paths encompassing that experienced by soils during cavity expansion. Specifically, the similarity technique is used to solve the cavity expansion problem in speswhite kaolin and Kurnell sand. Eight governing equations are defined and solved simultaneously as an initial value problem including an equilibrium equation for stresses around the cavity. Cylindrical and spherical cavities are considered, as are constant suction and constant moisture content conditions. Substantial differences in the stress-strain response of saturated and unsaturated soils surrounding expanding cavities are observed. The paper highlights the major influence of suction and the importance of accounting for this when using cavity expansion theory to interpret results of the cone penetration and pressuremeter tests.