In 2001, Illinois began administering the ACT (American College Testing) test to all 11th graders in Illinois public schools as part of the Prairie State Achievement Exam (PSAE). For the first time, ACT scores and background information were available for most public-school students who would be completing high school one year later, rather than just those who elected to take the test because they were expecting to apply to a postsecondary institution that asked for ACT scores. The authors refer to this cohort of public high school graduates as the Illinois Class of 2002. This report focuses on the relationships between college readiness and students' enrollment and completion patterns during the first four years of this longitudinal study, through Summer 2006. The Illinois Education Research Council (IERC) college readiness index combines information on students' ACT scores and high school grades into five levels--from not/least ready, through minimally ready, somewhat ready, more ready and most ready. Taken together, these data illustrate that more ready and most ready students from our cohort both enrolled in postsecondary education and completed certificates and degrees at higher rates than students from the somewhat, minimally, and not/least ready groups. Overall, more ready and most ready completers were more likely to complete baccalaureate degrees, while minimally and not/least ready completers were more likely to earn certificates or associate degrees. (Contains 16 figures and 2 tables.)