In every part of the world, already thousands of years ago, humans have speculated about some kind of life force. In China it is called qi (also spelled chi), in India prana, in ancient Greece pneuma, in Latin spiritus, and in Hebrew ruach. There are hundreds of life energy beliefs, which have many similarities. This encyclopedia presents and explains them all, showing their similarities, but also their differences. In the introduction of the book, distant origins of human thought are traced, examining how spiritual beliefs and life force ideas emerged, and how they were developed through time. Although the actual beliefs differ around the world, there are several common traits in how they were reached. The history of science also contains many examples of life force and vitalism speculations, often quite elaborate and making much sense at the time of their emergence. Several of the greatest scientific minds in history nourished such theories. They are included in the book. The book also covers many modern theories about a life force or similar concepts, stemming from scientific disciplines like physics, biology, and psychology, but also from alternative movements, such as New Age and religious societies.