There are some basic requirements for air conditioning systems in operating theatres. 1. Obviously the regulation of temperature and humidity is necessary, but cleaning of fresh air and circulating air is much more important. 2. Concerning cleaning, a high standard of filtration of air is demanded to avoid contamination of wounds. 3. So far only HEPA-filters, a technical description, give security and safety. They filter particles down to under 1 micrometer and have a filtration rate of more than 99.9%. 4. The ventilation rate is directly related to the germ concentration in the air. The higher the ventilation rate the cleaner the air during working. 5. To avoid turbulences unidirectional or uniform flow is necessary. This technique has become popular as the laminar flow technique. To get such a flow an air flow velocity of 0 45 +/- 0.1 m/s is necessary. 6. The arrangement of the different working places in an operating theatre is very important. Instrument tables and the operation field itself have to be in the zone of first air. 7. One of the commonest germ dispersion sources is the human body. Proper dressing and behaviour have to be observed. The airflow behind a person (in laminar cross flow) or under a person (in laminar down flow for instance when the neurosurgeon bends his head over the operating field) has to be regarded as contaminated. 8. Hygienic controls must be made in the operation theatre when unused and also under working conditions, in a sophisticated manner. If all these points are respected a real clean room technique is possible with considerable lowering of the risk of infection for the patient. Obviously all the other rules of hygiene, i.e., asepsis and antisepsis, elaborated since the days of Lister, have to be strictly respected.