The Lecture-Tutorial curriculum development project produced a set of 29 learner-centered classroom instructional materials for a large-enrollment introductory astronomy survey course for non-science majors. The Lecture-Tutorials are instructional materials intended for use by collaborative student learning groups, and are designed to be integrated into existing courses with conventional lectures. These instructional materials offer classroom-ready learner-centered activities that do not require any outside equipment or drastic course revision for implementation. Each 15-minute Lecture-Tutorial poses a sequence of conceptually challenging, Socratic dialogue-driven questions, along with graphs and data tables, all designed to encourage students to reason critically about difficult concepts in astronomy. The materials are based on research into student beliefs and reasoning difficulties, and use proven instructional strategies. The Lecture-Tutorials have been field-tested for effectiveness at various institutions, which represent a wide range of student populations and instructional settings. In addition to materials development, a second effort of this project focused on the assessment of changes in students’ conceptual understanding and attitudes toward learning astronomy as a result of both lecture and the subsequent use of Lecture-Tutorials. Quantitative and qualitative assessments were completed using a precourse, postlecture, and post-Lecture-Tutorial instrument, along with focus group interviews, respectively. Collectively, the evaluation data illustrate that conventional lectures alone helped students make statistically significant—yet unsatisfactory—gains in understanding (with students scoring at only the 50% level postlecture). Further, the data illustrate that the use of Lecture-Tutorials helped students achieve statistically significant gains beyond those attained after lecture (with students scoring at the 70% level post-Lecture-Tutorial). Quantitative evaluation of student attitudes showed no significant gains over the semester, but students reported that they considered the Lecture-Tutorials to be one of the most valuable components of the course.