Black flies are insects in the family Simuliidae which are blood-sucking pests of humans, domestic animals, and wildlife. They are known globally for disease transmission, but also for their vicious swarms which have resulted animal deaths from exsanguination. This chapter explores the biology and disease ecology of black flies, particularly onchocerciasis (river blindness). Lastly, diagnosis, laboratory findings, treatment and prevention of river blindness and its vectors are presented and discussed.