This book provides an up-to-date account of the precise experiments that can be performed in a terrestrial laboratory and are used to explore the nature of universal gravitation. The experiments required are at the limits of sensitivity of mechanical measurements. The problems of experiment design are discussed, and critical accounts given of the principal experiments testing the inverse square law and the principle of equivalence, and measuring the constant of gravitation. An analysis of the effects of noise and other disturbances is also provided, further highlighting the care that is needed in experimental design and performance. The motivation for undertaking such experiments is also discussed.