Fleas are insects in the order Siphonaptera which are bloodsucking ectoparasites of many mammals and birds worldwide. Some species are significant public health pests and disease transmitters. This chapter explores the biology, ecology, and disease relationships of fleas, including discussion of the primary flea species and where each breeds, what animal hosts they feed on, and the like. Lastly, flea-borne diseases such as plague, murine typhus, and flea-borne spotted fever are presented and discussed, including their geographic distributions, flea vectors, and comments about their treatment, prevention, and control.