The Early Childhood Longitudinal Study (ECLS) tracks children throughout the country as they move from kindergarten through fifth grade. As part of the ECLS planning process, the possibility was considered of assessing the program environments of Head Start children before they entered the ECLS kindergartens. A review was conducted of selected large-scale studies of Head Start, Chapter 1, child care, and other preschool settings to ascertain the important dimensions of children's program experience and to recommend ways of measuring those dimensions. This paper reviews those dimensions and measures of early childhood program environments that could be used in studies of preschool program effects on children's development. Following an introduction, chapter 2 of the review defines the important dimensions of program environments, drawing largely from research on early care and education program quality and from Head Start practices as reflected in the Head Start Performance Standards and performance measures. The five dimensions discussed are (1) classroom dynamics; (2) classroom structure; (3) classroom staff characteristics; (4) administration and support services; (5) parent involvement. Existing measures are also described. Chapter 3 summarizes findings from the 11 studies in which these measures have been used and suggests implications for future research. Chapter 4 presents recommendations for measures to use in future research. Two appendices include a summary of selected studies of Head Start and other early childhood program environments and descriptions of observational instruments. (HTH)