Guidelines are presented for incorporating screening and assessment into a sequential and comprehensive model of preschool intervention services. At-risk children who may receive intervention services and an ecological intervention model are described. Three categories of children may benefit from services of early intervention programs: children who are biologically at risk because of organic problems; children who are suspected to be at risk because of organic difficulties often associated with later problems; and children who are environmentally at risk because of social and interactional influences. An ecological model of early intervention services consists of a series of comprehensive steps: (1) case finding; (2) screening; (3) comprehensive assessment; (4) development of an intervention plan; (5) implementation of the intervention plan; (6) program monitoring and follow-up; and (7) development and implementation of a transition plan. Among the technical issues in planning and evaluation are the limitations of standardized tests in terms of reliability and construct validity. The criteria used to make decisions from screening and assessment must incorporate information about referral and eligibility rates and the validity of decisions. Five tables present information from previous studies to illustrate the discussion. (SLD)