A multiple constituencies model of human service organizations identifies twelve interest groups which must be considered when effectiveness questions are raised. The differential power of the interest groups suggests that some groups' preferences are likely to be emphasized over others. The relationship between power inside the organization and that on the outside is analyzed. Recent trends in the growth andmiiitancy of professional associations and employee groups suggest that internal control by senior administrators is increasingly challenged and variable. Future studies of effectiveness in the human services are encouraged to remain sensitive to the effects of constituency interests and power on the establishment and implementation of priorities and goals.