The design process for an energy conscious building, which was built, along with the computer aided design tools that were applied, is presented. The building, situated in the hot-humid climate of Rehovot - Israel, houses the laboratories and offices of the Weizmann Institute＇s Environmental Science and Energy Research Department. Alternative bio-climatic design options were proposed and evaluated throughout the detailed design stage. A building energy performance index (BEPI) was established for each alternative. This index reflects the total amount of energy consumption for heating, cooling, ventilating and lighting used, per square meter of floor area. Thermal modeling for the different design alternatives were carried out by means of an hourly dynamic simulation model ENERGY. The design strategies were determined by the model PASYS, the shading devices were designed and evaluated using SUNSHADES and SHADING design tools. The building is monitored for the last whole year, and the measured values are close to the predictions.