Several studies have examined homophobia in the general population and in different groups of helping professionals such as social workers, counselors, psychologists, and nurses. However, few scales have been developed and validated to assess how practitioners interact with gay and lesbian clients in clinical settings. To address this deficit in the literature and to examine the degree to which practitioners engage in gay affirmative practice, this dissertation focused on the development and validation of the Gay Affirmative Practice Scale (GAP). Gay affirmative practice is a model of practice with gay and lesbian clients which treats them as equal to heterosexuals and considers their identities in the context of the oppression they experience. This study consisted of four stages: 1) draft of an initial pool of items, 2) administration of the items to a pool of experts to assess the items’ content validity, 3) administration of the behavioral domain to assess which of two response sets yielded the greatest variability in responses, and 4) administration of the scale to clinicians to assess the reliability and validity of the instrument. The final version of the GAP consists of two 15-item domains with strong evidence for their reliability and validity. Chronbach’s alpha for the belief domain is .9307 and for the behavior domain is .9375. Both domains have strong factorial validity with each item loading on its respective domain at .60 or greater. The scale has convergent construct validity, as demonstrated by significant relationships with measures of homophobia, and discriminant construct validity, as demonstrated by a non-significant relationship with a measure of social desirability. Significant relationships were found with GAP scores and measures of contact and feelings about gays and lesbians, training on gay and lesbian issues, current political party, sexual orientation, relationship status, primary role at agency, and primary practice area. Use of this scale in future studies can yield important insight into factors that influence the degree to which clinicians practice affirmatively with gay and lesbian clients. In doing so, it is hoped that treatment with gay and lesbian clients will improve.