Attempts to determine costs in the intensive care unit (ICU) were not successful until now, as they failed to detect differences of costs between patients. The methodology and/or the instruments used might be at the origin of this failure. Based on the results of the European ICUs studies and on the descriptions of the activities of care in the ICU, we gathered and analysed the relevant literature concerning the monitoring of costs in the ICU. The aim was to formulate a methodology, from an economic perspective, in which future research may be framed. A bottom-up microcosting methodology will enable to distinguish costs between patients. The resulting information will at the same time support the decision-making of top management and be ready to include in the financial system of the hospital. Nursing staff explains about 30% of the total costs. This relation remains constant irrespective of the annual nurse/patient ratio. In contrast with other scoring instruments, the nursing activities score (NAS) covers all nursing activities. (1) NAS is to be chosen for quantifying nursing activities; (2) an instrument for measuring the physician's activities is not yet available; (3) because the nursing activities have a large impact on total costs, the standardisation of the processes of care (following the system approach) will contribute to manage costs, making also reproducible the issue of quality of care; (4) the quantification of the nursing activities may be the required (proxy) input for the automated bottom-up monitoring of costs in the ICU.