The present article discusses the three major components of behavior that are used for assessing pain—self-report, overt behavior/function, and physical indices. Issues concerning the reliable operational definitions of these three behavioral referents of pain are reviewed, as well as the fact that a high degree of concordance or precise overlap among the three cannot be automatically assumed. The best biopsychosocial assessment approved to be used when discordance occurs—a stepwise approach to assessment—is then presented. Finally, in addition to the often complex interactions among the three behavioral referents of pain, what makes this assessment area even more complicated is that there may also be complex interactions among the construct of pain and the two related constructs of impairment and disability. These potential complexities are also discussed.