Leptonic decays of hadrons

The theory of semi-leptonic weak interactions is reviewed and confronted with present experimental data. The theoretical points emphasized are the basic tenets of the Cabibbo theory, the algebra of currents, and the relevance of strong interaction symmetries to the weak semi-leptonic amplitudes. The experimental data is discussed in detail, and future lines of investigation, necessary for testing detailed theoretical predictions and for resolving outstanding questions, are indicated.