When characterizing a differential-mode transmission path, it is usually assumed that the differential pair is balanced thereby simplifying the analysis. The theory and characterization of balanced pairs is well-established. By contrast, the theory of unbalanced differential pairs has not been extensively investigated despite the fact that such pairs appear in many applications. A complete generic model and characterization of unbalanced differential transmission line pairs is presented. Pairs having geometrical or impedance imbalance as well as pairs having temporal imbalance (also known as in-pair skew) are considered. The paper also describes how to accurately characterize unbalanced lines using TDR and VNA measurements.