Purpose: The extant literature suggests that more than 19% of social workers experience depression (Siebert, 2004), and that social workers may be at a higher risk than other professionals to develop problems in professional functioning (Deutsch, 1985; Elliott & Guy, 1993; Siebert, 2003; Strozier & Evans, 1998). Very little effort has been spent understanding how depression and problems with professional functioning might be prevented or mediated. The purpose of this study is to explain how the effect of depression on professional functioning (impairment) is mediated by self–esteem, organizational wellness, and social support. This paper reports on an explanatory study that examined these variables. Method: The cross sectional survey design utilized an anonymous internet questionnaire. A random sample of 1,250 licensed social workers in Kentucky was invited to participate via a mailed postcard. The internet survey was completed by 215 social workers, with a survey completion rate of 17%. Results: Using the K 10 (a=.88) to measure depression and anxiety, 3% of the sample had ten times the general population risk of having a depressive or anxiety disorder, and 39% of the sample had three times the general population risk of having a depressive or anxiety disorder. Using a strengths based measure of depression (energy level subscale, a=.90, of the Corporate Behavioral Wellness Inventory), found that 19% of social workers currently experienced depression. Using the Inventory of Professional Functioning (IPF, a=.92) to measure impairment, 11% of the sample reported significant problems in professional functioning. Linear regression revealed that depression accounted for 9% of the variance in impairment (r2=.09, F=20.64, p=.00). Testing for mediation using the Sobel test (Barron & Kenny, 1986) revealed that the effect of depression on impairment was completely mediated by self-esteem, organizational wellness, and social support. Implications for practice: This study's findings have important implications for preventing impairment. Prevention of the negative consequences of mental health problems on professional functioning should form a focal point within the profession. Impairment prevention could start as soon a person enters the educational process. Educators and researchers, and managers and supervisors should understand how depression affects professional functioning and find ways to enhance self-esteem, improve organizational wellness, and build social supports among its members and within organizations. Self-examination of individuals and the systems in which the profession functions is essential. A profession which is improving the wellness of its workforce may have better client outcomes.