Reduction in the size of classes from Kindergarten to Grade 3 was a major Liberal Party campaign promise in Ontario's 2003 provincial election. It was intended to demonstrate a new government's commitment to improving public education. By the 2008-09 school year, the provincial government's goals had been achieved: over 90% of all primary classes had 20 or fewer students, and "all" primary classes had 23 or fewer students. The provincial government has much to be proud of. The speed and totality of success in achieving primary class size (PCS) reduction has been remarkable. Through a contribution agreement with the Ministry of Education, the Canadian Education Association contracted with a team of researchers from the Ontario Institute for Studies in Education in 2007 and 2008 to conduct the study that resulted in this report. The report is based on reviews of the research on class size reduction, analyses of statistical data collected by the Ministry of Education over a five-year period, and research across the province in eight school boards, 24 schools and 84 classrooms. This report focuses on the benefits and consequences of Ontario's primary class size reduction initiative in its early days. Its primary purposes are to reveal strengths and challenges and to provide recommendations that will increase the potential of PCS to achieve provincial goals for high-quality education. Two appendices are included: (1) Research Methodology; and (2) Consent Letters and Interview Protocols.