The original Air Engines (also known as a heat, hot air, caloric, or Stirling engines), predated the modern internal combustion engine. This early engine design always had great potential for high efficiency/low emission power generation. However, the primary obstacle to its practical use in the past has been the lack of sufficiently heat-resistant materials. This obstacle has now been eliminated due to the higher strength of modern materials and alloys. Several companies in the U.S. and abroad are successfully marketing new machines based on the Air Engine concept. Allan Organ and Theodor Finkelstein are two of the most respected researchers in the field of Air Engines. Finkelstein is considered a pioneer of Stirling cycle simulation. The historical portion of the book is based on four famous articles he published in 1959. The rest of the chapters assess the development of the air engine and put it in the modern context, as well as investigate its future potential and applications.