The necessity for a safe and comfortable environment is older than the human species. Birds build nests, rabbits dig holes. Early human societies succeeded, in some cases, in creating remarkably pleasant accommodations. Caves shows that under an overhang to block the summer heat, yet accessible to the warming rays of the winter sun, with the massive heat capacity of the surrounding rocks. Those are the first realization of the principles of passive solar architecture. The ancient people were quite conscious of the benefits to be obtained by good orientation of a building with respect to the sun, and they laid out entire settlements facing south. The word comfort was achieved from the people who built them first central heating, with a double floor through whose cavity the fumes of a fire were passed. The first transparent window coverings were made of materials such as mica or glass. Thus it became possible to admit light into a building without letting in wind, rain, or snow.it took a long time before buildings reached what we would consider com¬fortable conditions. When visiting castles and ancient monuments built in the sixteenth century and looking at their heating arrangements, we can guess how cold winter must have been even for the rich people. Heat is a necessity to humanity. Without an adequate heating system, many of our effort would be eliminated. Nothing can distract a person faster than being cold. Everything contains heat energy; cold is the absence of heat. If you are exposed to extreme cold, you first feel distracted; then feel extreme discomfort; and then enter a potentially permanent sleep. Early pictures show cave dwellers huddled around a fire trying to keep warm. Fire, one of the first heat sources, has been, and still is, an excellent heat source. The fireplace was a usual heating system in years past. Many homes still use the fireplace as a primary heat source Fireplaces cause concern because of factors such as pollution, lack of efficiency, and the user's labor. However, the main disadvantage of the fireplace is its efficiency factor. Some people developed methods that improved fireplace efficiency. Later inventors developed heat exchangers that substantially improved fireplace efficiency. Fireplaces have been replaced by central heating systems, which have become the basic heat source in most homes today. Cooling is more difficult than heating. In the past the basic method was to ward off the sun, coupled with the use of heavy stonework for thermal inertia—actually quite effective in climates with cool nights. Finally architects of castles found the solution with proficient use of running water, providing some evaporative cooling. In certain parts of the world, some build¬ings take advantage of cool breezes that blow regularly from the same direction. Nights used to be dark because candles and oil lamps were expensive, to say nothing of the quality of the light. In fact before the invention of electric lights, access to sunlight was a main criterion for the design of buildings. Sunlight does not penetrate well into the interior; as a rule of thumb, adequate illumination on cloudy days cannot be provided to depths beyond 1.5 to 2 times the height of the upper window edge. This, together with the need for fresh air before the days of mechanical ventilation, explains the shape of buildings well into the first half of the twentieth century: No room could be far from the perimeter. Effective air conditioning had to await the development of mechanical refriger¬ation during the first decades of the twentieth century. Routine installation of central air conditioning systems dates from the 1960s.