Angular Momentum Theory for Diatomic Molecules focuses on the application of angular momentum theory in describing the complex dynamical processes in molecules. The manuscript first offers information on tensor algebra and rotation group. Discussions focus on commutation relations, spherical and double tensors, rotations, coupling, reduced matrix elements, quaternions, combination theorem for Gegenbauer polynomials, and combination theorems for spherical harmonics. The book then takes a look at R(4) in physical systems and hydrogen molecular ion, including rigid rotator, reversed angular momentum, reduced matrix elements, spheroidal coordinates, and hydrogen atom in spheroidal coordinates. The publication examines expansions and free diatomic molecules. Topics include angular momentum, molecular frame, primitive energy spectrum, rotating oscillator and hydrogen atom, expressions for electric potentials, delta functions, and Neumann expansion. The manuscript also considers external fields and perturbations. The text is a dependable reference for readers interested in the application of angular momentum theory in identifying the dynamical processes going on in molecules.