The problems inherent in the estimation of fracture mechanics parameters for three-dimensional crack problems are reviewed. A novel parameter is introduced to quantify the loss of constraint along that portion of a crack front adjacent to a free surface. Finite element analyses of a compact tension specimen with a straight crack front indicate the usefulness of this parameter in studying the influence of Poisson's ratio and plate thickness on the extent of the free surface boundary layer effect. A method for predicting the shape of a natural crack in this specimen is also presented.