This article, the first of two parts, discusses the implementation of 12-hour shifts using a locally devised nursing development unit (NDU) framework. A literature review and force-field analysis were undertaken to plan, implement and evaluate the introduction of the 12-hour system. The literature review identified five broad categories: effect on care delivery; nurse education; cost-effectiveness; impact on staff; and implementation strategies. It also ascertained that the most successful methods of implementation were those that gained the cooperation of staff. The force-field analysis identified the restraining forces (e.g. tiredness, the European Working Time Directive and staff views) and therefore allowed the change agent to concentrate his limited time on combating these restraining forces. The second part of this series will examine the evaluation and audit of the 12-hour shift system.