The first of two reports of a national evaluation of the immediate effects of Project Head Start describes the characteristics of children, families and programs in samples of full-year classes operating in 1968-69. The study identifies changes associated with Head Start participation and the conditions under which these changes were greatest. No control groups were used; comparisons were made within the Head Start sample to see what kinds of classroom experiences "work best" for what kinds of children. Chapters deal with: (1) Head Start and early childhood education; (2) research and evaluation studies of Head Start and other early childhood programs; (3) designs of the three years' evaluations; (4) limitations in the studies; (5) measures for 1968-69 analysis--program and performance variables; (6) the children: entering characteristics; (7) their families entering characteristics; (8) the programs--teachers, classrooms, activities; (9) gains associated with Head Start--cognitive and social-emotional measures and parent attitudes; (10) differences in gains for different types of children; (11) differences in gains associated with different program approaches; (12) relationships between program approaches and performance for different subgroups of children; (13) summary and conclusions; and (14) executive summary. Appendices provide locations and directions of Head Start E & R centers, more information about variables, and frequency distributions on dependent variables. References are provided. (KM)