The National Institute of Child Health and Human Development (NICHD) Study of Early Child Care is an ongoing, prospective, 3-year longitudinal study of over 1,300 full-term healthy infants and families from 10 sites across the United States. While the sample is not nationally representative, the subjects come from major regions of the country: the East Coast, the West Coast, the Mid-West, the North, and the South. They come from urban and from rural settings, and include minorities as well as single-parent and two-parent families. The study examines the concurrent, long-term, and cumulative influences of variations in early child care experiences on the cognitive, linguistic, social, emotional, and physical development of infants and toddlers. The study design takes into account the complex interactions among characteristics of the family and home, of the child care environments, and of the child. Subject recruitment started in January 1991, and data collection was scheduled to end at the beginning of 1995. Data collection involved periodic telephone calls to families beginning at 3 months and home visits, child care center visits, or laboratory assessments at 1, 6, 15, 24, and 36 months. This document is volume 4 of a 5-volume operations manual that explains, in 29 chapters, the procedures used in conducting the NICHD study. Volume 4 consists of chapters 20 through 24, which describe the 24-month visits and assessments, except that the volume begins with chapter 13 reproduced from volume 3. This chapter provides instructions for conducting follow-up telephone interviews at 13 and 23 months. Chapter 20 presents an overview of the visits to the child's home and child care center, and laboratory assessments at 24 months. Chapters 21 and 22 outline the procedures to follow in the visits to the child's home and child care center at 24 months. Chapter 23 provides instructions for: (1) conducting the 24-month laboratory visit; (2) observing and coding children's solitary play, and assessing children's compliance during a clean-up activity; (3) administering the Bayley Scales of Infant Development; (4) videotaping mother-child interactions; (5) conducting standardized interviews and questionnaires; and (6) measuring children's physical growth. Chapter 24 presents instructions on making follow-up telephone calls at 27, 30, and 33 months. (BC)