Community colleges across the United States face a difficult challenge. On the one hand, they are "open access" institutions, with a mission to serve students from all backgrounds and at varying levels of college readiness. On the other hand, they must uphold high academic standards in order to maintain accreditation and prepare students for employment or transfer to four-year schools. How, then, can community colleges best serve students who want to learn but do not meet minimum academic standards? Chaffey College, a large community college located about 40 miles east of Los Angeles, began to wrestle with this question early in the twenty-first century. Under the auspices of a national demonstration project called Opening Doors, Chaffey developed a program designed to increase probationary students' chances of succeeding in college. Chaffey's program included a "College Success" course, taught by a counselor, which provided basic information on study skills and the requirements of college. As part of the course, students were expected to complete five visits to "Success Centers," where their assignments, linked to the College Success course, covered skills assessment, learning styles, time management, use of resources, and test preparation. In 2005, MDRC collaborated with Chaffey College to evaluate the one-semester, voluntary Opening Doors program. In 2006, the program was improved to form the two-semester Enhanced Opening Doors program, in which probationary students were told that they were required to take the College Success course. In MDRC's evaluation of each program, students were randomly assigned either to a program group that had the opportunity to participate in the program or to a control group that received the college's standard courses and services. This report presents the outcomes for both groups of students in the Enhanced Opening Doors evaluation for four years after they entered the study. The findings include: (1) The message matters--optional program activities had lower participation rates compared with required program activities; (2) Chaffey's Enhanced Opening Doors program had positive short-term effects; and (3) Despite the program's encouraging short-term effects, it did not meaningfully improve students' long-term academic outcomes. This report presents detailed findings from Chaffey's Enhanced Opening Doors initiative, including the cost and cost-effectiveness of the program, and considers the implications of this research for designing services for probationary students in community college. Appended are: (1) Sample Characteristics at Baseline, by Research Group, and Supplementary Four-Year Impact Tables; (2) Measure Creation; and (3) Statistical Model for the Impact Analysis. Individual chapters contain footnotes. (Contains 20 tables, 7 figures and 1 box.) [Additional funding for this paper was provided by the MacArthur Foundation Research Network on Socioeconomic Status and Health and the MacArthur Foundation Research Network on Transitions to Adulthood. For "Serving Community College Students on Probation: Four-Year Findings from Chaffey College's Opening Doors Program. Executive Summary," see ED526394.]