Influencing the ordering of laboratory investigations, and in particular reducing unnecessary ones, is a vital concern to many hospitals. An account of the reasons why control is necessary is presented. The different methods evaluated are reviewed, including rationing, form design, resource management (budget control), education, protocols and decision support systems, incentives, feedback and case note appraisal. Methods of controlling the requesting of investigations are likely to be ineffective unless they take place in the correct environment. Elements of this include correct attitude and commitment by senior staff, a long term strategy, and different approaches for different groups of doctors.