As a typical sap-feeding insect, aphid requires a primary symbiont, Buchnera aphidicola, which can synthesize and provide some essential nutrients to the host. In addition, aphids may also be infected with one or more additional bacteria, called secondary or facultative symbionts that are mutualistic for various ecological interactions. Recently research on pea aphids has found that facultative symbionts protect against entomopathogenic fungi and parasitoid wasps, reduce detrimental effects of heat, and influence host plant suitability. Based on these results, we reviewed the literature relative to facultative symbionts of aphids and summarized the variety and modes of transmission of facultative symbionts in aphids, phenotypes associated with aphid facultative symbionts, and genomics of aphids symbionts to provide a baseline for the research of additional bacteria.