An analysis of general scalar-tensor gravitation theory, containing two arbitrary functions of the scalar field, is presented. The weak-field limit is considered in detail, and predictions for the classical tests of gravitation theory are derived. A definite relationship between the light propagation and perihelion shift effects is found to hold under very general conditions. The theory of the detection of gravitational waves is also investigated, and the observable differences between the scalar and tensor components are indicated. Finally, the relationship between the properties of the source and its radiation is considered in the weak-source limit, and expressions for the rate of energy loss are derived. It is shown that the existing observational data are consistent with the possibility that the scalar field represents a major component of gravitational radiation from astronomical sources.