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A non-contacting computer vision method, Digital Image Correlation (DIC), that accurately quantifies full-field surface deformations is described. A chronology of the development of both two- and three-dimensional DIC methods is given. The two-dimensional DIC results are presented for (a) elastic-plastic crack tip fields both before and during crack growth, (b) crack tip opening angles during stable crack extension and (c) measurements of surface strains at temperatures as high as 650 degree C. Results from applying the three-dimensional DIC method to the study of surface elastic-plastic crack tip fields in compact tension specimens before crack growth are presented. Included are discussions of both error sources and methods for error minimization.