After an extraordinary technical development, medicine is under close scrutiny and requested to prove that its diagnostic and therapeutic procedures are "efficient, appropriate, and economical". Evidence based medicine allows the optimal use of existing data in the literature, but focuses only on health care benefits. Economical analyses take into account the health care resources needed to get these benefits. The various types of costs, perspectives of analysis, and techniques for assessing benefits and uncertainty about cost estimates, are presented and illustrated with two examples drawn from the fields of primary care and advanced technology. Because the cost benefit ratio of a diagnostic or therapeutic procedure is heavily dependent on the type of patients to which it is applied, as well as on the stage of technological progress, the two types of information are necessary to fully assess medical procedures.