Experimental results are presented which suggest that parameters based on the J-integral and the crack opening tip displacement δ are viable characterizations of crack initiation and stable crack growth. Observations based on some theoretical studies and finite-element investigations of the extending crack revealed that J and δ when appropriately employed do indeed characterize the near-field deformation. In particular, the analytical and experimental studies show that crack initiation is characterizable by the critical value of J or δ, and stable crack growth is characterizable in terms of the J or δ resistance curves. The crack opening angle, dδ/da, appears to be relatively constant over a significant range of crack growth. Thus, appropriate measures of the material toughness associated with initiation are J_Ic and δ_Ic, and measures of material toughness associated with stable crack growth are given by the dimensionless parameters T_J [= (E/σ_o²)(dJ/da)] and Tδ [= (E/σ_o)(dδ/(da)]. The two-parameter characterization of fracture behavior by J_Ic and T_J or δ_Ic and Tδ is analogous to the characterization of deformation behavior by the yield stress and strain hardening exponent.