A history of the attempts to test the predictions of Newtonian Gravity, describing in detail recent experimental efforts to verify both the inverse-square law and the Equivalence Principle. Interest in these questions has increased in recent years, as it has become recognised that deviations from Newtonian gravity could be a signal for a new fundamental force in nature. This is the first book devoted entirely to this subject, and will thus be useful to both graduate students and researchers interested in this field. It describes the ideas that underlie searches for such deviations, focusing on macroscopic tests. A comprehensive bibliography of some 450 entries supplements the text.