AbstractThis paper presents a systematic study of the theories of gravitation that have been proposed. We restrict ourselves to an investigation of the linear approximation since new connections with experiment can be expected in this order in χ only.It turns out that all theories that differ from general relativity contain a scalar field and describe gravitation by a mixture of scalar and tensor interactions. The scalar has negative energy in theories with light deflection larger than the Einstein value. As a consequence instabilities of cosmic systems and continuous creation result in this case. Furthermore, the emission of gravitational radiation with negative energy could be the energy source of the quasars.The question of the mass renormalization of the scalar is investigated. No unique answer is possible at present.Finally a new class of gravitational theories is given