Models of the small-scale structures in radio sources are limited by the resolution, dynamic range, and sensitivity of the observations upon which they are based. However, evidence from variability, brightness, and proper motion observations strongly support the contention that these objects involve directed outflow or ejection of material at relativistic speeds. The evidence for relativistic motion will be discussed, and a variety of models involving both bulk and pattern motion will be analyzed in terms of spatial and temporal characteristics which may distinguish between models. The possibilities and hazards of generalizing nonrelativistic jet models to the relativistic case will also be briefly discussed, especially regarding possible observational signatures of magnetized versus unmagnetized jets.