This article describes the development and initial psychometric evaluation of a measure of individual appraisal of readiness to initiate health behavior change. Items were developed from inductively generated data to index dimensions of individual appraisal of readiness. Quantification of Index of Readiness content validity was established through the ratings of 10 experts, following criteria established by Imle and Atwood (1988). The instrument was tested in successive steps with 146 individuals who were participating in an outpatient cardiac rehabilitation program, for reliability and validity, including internal consistency reliability, and three forms of validity assessment (content validity, criterion-related validity, and construct validity). The three subscales of Reevaluation of Lifestyle, Identification of Barriers, and Goal Commitment demonstrated internal consistency. Criterion-related and construct validity were substantiated. With refinement, this measure will provide a basis for nursing interventions designed to enhance individual motivation in relation to specific health behaviors.