In this study, the methods used by both industry and paper physicists for evaluating paper toughness (resistance to breakage) are critically reviewed, and a new method for determining the critical value of the J-integral, J, is presented. Difficulties arising from the use of tensile strength and tensile energy absorption in the evaluation of paper toughness are highlighted. J,, values obtained with the Leibowitz non-linear technique were relatively close to those obtained with a new method developed by the authors when the latter was used in conjunction with key-curve analysis for determining the critical point. This result suggests that the Liebowitz non-linear technique may give a relatively accurate J integral value at the crack initiation point with less experimental effort. Experimental results show that it may also be possible to use notched specimens in the standard tensile testing configuration for J-integral estimation, which would be an attractive method for industry.