The field-dependent spin reorientation transition (SRT) in ultrathin ferromagnetic films is discussed. A rather general treatment is presented which involves the extension of the anisotropy-flow concept to systems in an applied magnetic field. Special features of field-induced SRT's are deduced from the general analysis. Emphasis is laid on the experimental implications, whereby general features are quantitatively described which are as characteristic as fingerprints and serve to set up a natural classification of the SRT's in external fields. Special attention is dedicated to resolving the substantial differences between in-field and spontaneous SRT's; ignoring these differences may result in grave misinterpretations of experimental findings