Compared with the traditional focus on developmental problems, research on positive development is relatively new. Empirical research in children's well-being has been scarce. The aim of this study was to develop a theoretically and empirically based instrument for practitioners to observe and assess preschool children's well-being in early childhood settings. The analysis of preschool teachers' ratings yields six dimensions of social-emotional well-being: (1) making contact/social performance, (2) self control/thoughtfulness, (3) self-assertiveness, (4) emotional stability/coping with stress, (5) task orientation, (6) pleasure in exploration. Composite scales were constructed. PERIK consists of six scales of six items each. The scales differentiate in both the upper and lower range and despite their brevity have good psychometric qualities. The instrument was published together with a booklet containing examples of how PERIK observations can be employed in practical work with children. (Contains 2 tables and 3 notes.)