We use an analytical two-band model to derive material descriptors for predicting TE performance of narrow-gap semiconductors and semimetals. We find that the material descriptors that govern TE performance are: (1) the nand p-type quality factors ( ), (2) the asymmetry parameter R , which is the ratio of the quality factors, and (3) the band gap. As in a single-band model, materials with high quality factors tend to exhibit high TE performance when doping is optimized. However, high TE performance in narrow-gap semiconductors and semimetals also hinges on suppressing bipolar conduction e↵ects. Bipolar conduction increases the overall electrical conductivity, but it also decreases the Seebeck coe cient and increases electronic thermal conductivity -both detrimental to achieving high zT . Our two-band transport model shows that bipolar conduction e↵ects can be suppressed when electron and hole transport properties are asymmetric. In terms of material descriptors, this translates to a high R or 1/R .