A general phenomenological discussion of spin reorientation transitions (SRT's) is given which contributes to the understanding of both bulk and thin-film behavior in applied fields. The two principal field configurations (parallel and perpendicular to the crystallographic axis of symmetry) are considered. In contrast to spontaneous SRT's, only two phases compete when an external magnetic field is applied, a canted and a field-aligned one. The complete stability analysis leads to a basic phase diagram which encompasses both configurations at once. The coexistence of phases is seen to persist even with the field and is described quantitatively. The practical implications of the analysis for temperature-driven and thickness-driven SRT's are carefully considered.