Montessori education is the oldest and most widely implemented alternative education in the world and includes a full system of lessons and hands-on materials for children from birth to 18 years, presented individually, and embedded in a philosophical framework regarding children's development and its optimal conditions. The objectives of this review were to examine the effectiveness of Montessori education in improving academic and nonacademic outcomes compared to traditional education, and to determine the degree to which grade level, Montessori setting (public Montessori vs. private Montessori), random assignment, treatment duration, and length of follow-up measurements moderate the magnitude of Montessori effects. Relevant studies were searched in 19 academic databases, in a variety of sources known to publish gray literature, in Montessori-related journals, and in the references of studies retrieved through these searches. This review found that Montessori education had a significant positive impact on academic and nonacademic outcomes. Studies with random assignment, elementary school age level, and private Montessori schools had larger effects.