This paper describes school improvement through clusters (or leagues) of the Colorado Department of Education. School improvement clusters are defined as associations of schools and cooperating organizations dedicated to improving the quality of education. Participants work together with a common goal or unifying concept. The paper describes 14 characteristics of effective cluster groups. Important points are: membership is limited to five to eight schools in close proximity to each other, and school participation is committed for at least 3 years. Principal and teacher-leader roles in the school cluster are important; eight specific responsibilities are outlined. Each school maintains an accountability committee and a school improvement management team. The superintendent plays a supportive role as a resource for the cluster groups. Each cluster has a "hub," a group of specialists from participating organizations that provides support services and leadership to the cluster members. The paper includes a diagram of the Colorado Educational Improvement Network. (MD)