The sterilization requirements for medical/pharmaceutical applications are traditionally based on an extensive overkill. In the past few years, however, an evolution towards bioburden related sterilization processes has been started (F0 theory). Especially manufacturers of large volume parenterals have - forced by the thermolability of the product - contributed to this development. In this paper both philosophies are combined, resulting in a concept in which the bacteriological and physical bases of the sterilization process are mathematically related by using the F0 theory and by introducing an Imaginary Micro-Organism (IMO). The IMO concept provides the opportunity for anyone in the field of sterilization to raise the quality control level, which can be achieved by: - selecting optimum sterilization conditions without performing pre-sterilization counts: - step by step introducing the pre-sterilization count which results in even more favourable sterilization conditions.