The impact of the urban environment on natural and hybrid ventilation was investigated through experimental and computational procedures in the framework of RESHYVENT European Project. An experimental campaign was organized in two urban street canyons in Athens, during summer 2002, consisting of field and indoor experiments. The experiments aimed at the investigation of the impact of the various urban features on the efficiency of different ventilation systems. Natural, mechanical and hybrid ventilation experiments were carried out in three building apartments under different ambient conditions. A comparison analysis, in terms of air exchange rates and air-exchange efficiency, was performed between the studied ventilation systems. Besides, computational calculations of air flow characteristics were carried out for advanced RESHYVENT hybrid ventilation systems considering a great number of simulation scenarios. The most important constraints of the urban environment on the performance of hybrid ventilation systems are highlighted.