The normal stress ratio theory for crack extension in anisotropic materials is studied analytically and experimentally in order to evaluate its validity. The theory is applied within a macroscopic-level analysis of a single center notch of arbitrary orientation in a unidirectional composite material. The bulk of the analytical work of this study applies an elasticity solution for an infinite plate with a center line crack to obtain critical stress and crack growth direction predictions. An elasticity solution for an infinite plate with a center elliptical flaw is also used to obtain qualitative predictions of the location of crack initiation around the border of an actual rounded notch tip. The analytical portion of the study includes the formulation of a new crack growth theory that includes local shear stress. Predictions of the normal stress ratio theory are obtained for the problems of a unidirectional tensile coupon with a horizontal center notch and a unidirectional losipescu shear specimen with a vertical center notch, each with an arbitrary fiber orientation. These predictions are subsequently compared to experimental results. It is shown that the normal stress ratio theory exhibits a strong ability to correctly predict crack extension direction. Predicted critical stresses correlated well with experimental stresses at crack initiation. Use of the elliptical flaw analysis resulted in significant agreement with observed locations of crack extension, while still providing correct crack extension direction predictions. It is suggested that future analytical studies include application of the normal stress ratio theory as a predictor of critical stresses and its application within a rounded notch tip analysis. Also, future experimental efforts should include performing the critical shear tests identified in this study which could not be performed using the Iosipescu specimen. / M.S.