The theory of homonuclear decoupling combined with magic angle spinning is developed in the average Hamiltonian formalism. A description of pulse sequences used to tune for pulse width, pulse phase difference, and to minimize phase transient impurities is given. Resolution of n.m.r. spectra of 1 H in crystalline, partly crystalline and amorphous solids under various pulse sequences with and without magic angle spinning are illustrated. Sources of residual broadening are discussed.