Energy conservation in buildings is an urgent issue to prevent the global climate change because one third of energy is consumed in the building sector in most countries.In particular, as almost a half of energy is used by air-conditioning systems or HVAC systems in commercial buildings, its energy conservation is a matter of high priority.In most countries improvement of the energy efficiency of the system is regarded as a critical design process which is often prescribed by the building standards or codes.However, in real operation of the system a great deal of discrepancy can be often found between predicted or designed, and measured values.Accordingly improvement or optimization of the system performance in the course of building use is becoming a very important task.However in case of a large and sophisticated energy plant such as a district heating and cooling system or a new type of low energy buildings which often use renewable energy such as ground source thermal energy, optimization based on a rule of thumb is hardly successful because of the sophistication beyond human perception in addition to lack of experience.To cope with this problemthe author believes that the effectiveness of utilization of model based optimization should receive more attention.In the present paper explanation of the background, methodologies and future issues in terms of optimization of HVAC syste.ms are given by showing real examples and results on the commissioning framework.