The central theme of this paper is that the potential for effective teaching is significantly increased by a teacher's exploring and employing a pupil's social background knowledge. This paper identifies some theoretical assumptions, describes categories and approaches of social diagnosis, and offers teaching applications. The concept of social background derives from the view that each person has a unique social history of events and influences. Important social and educational variables can be organized into six general diagnostic categories: (1) Schooling background; (2) Family background; (3) Physical health; (4) Social Skills; (5) Perceptual skills and organization (to determine the basic learning style of a pupil); and (6) Personal preferences. One of the most important informal methods is direct and unobtrusive observation of a pupil at different times and settings. Another approach is holding weekly conferences with each pupil. Dialogue with parents can be productive if they do not feel threatened. One of the most important aids in analyzing a pupil's social background is to devise some simple record keeping system. To respond to a pupil's diagnosed needs, a teacher should relate specific diagnostic data to specific instructional content, materials, and methods. Diagnostic factors are not excuses for a pupil's deficiencies; for diagnosis identifies problems to be attacked with new and concentrated remedies. (Author/AM)