Lice are insects in the order Psocodea (formerly Phthiraptera) which are ectoparasites of many mammals and birds worldwide. There are two main subgroups of lice—sucking lice and chewing lice. Sucking lice feed on blood, while chewing lice feed on fur, feathers, and dermal secretions. Some species of sucking lice are significant public health nuisance pests and disease transmitters. This chapter explores the biology, ecology, and disease relationships of lice affecting humans. Lastly, the diseases epidemic typhus, trench fever, and louse-borne relapsing fever are presented and discussed, including their geographic distributions and comments about their treatment, prevention, and control.