There are several examples of successes in improving health care. However, many of these remain limited to the sites at which they were originally developed. There are fewer examples of successful spread of the improvement more widely inside or outside the health systems within which they were developed. This article discusses the wave-sequence approach to spread or scale up, which enables take up of the improvement in a systematic and sequential way, using "spread agents" - people who participated in the original demonstration sites. The paper also discusses the concept of the "slice" of a system which is useful for thinking about spread and considers a phenomenon related to the rate of adoption which we have observed in this wave-sequence approach.