The purpose of this study is to determine the influences that affect the development of critical thinking skills in undergraduate students. This study examines data from the College Student Experiences Questionnaire (CSEQ), a national, cross-sectional survey completed by students during the 1992–1993 academic year. Using Astin’s I-E-O model and Pace’s quality of effort theory as the conceptual framework, the study examined students’ background characteristics, library experiences, experiences with faculty, course learning, and experiences with writing to determine the greatest influences on gains in critical thinking development. Results of the factor analyses and the hierarchical multiple regression indicated that grades, class year, focused academic library activities, informal faculty interaction, active course learning, and conscientious writing all resulted in gains in critical thinking. Implications of these findings suggest that academic librarians should design library research skills courses to increase students’ focused use of the academic library and its resources in order to increase their critical thinking skills.