The genus Streptococcus encompasses a broad range of colonizing and pathogenic organisms. Streptococci inhabit animals as well as humans and can cause infection in both. They are found most abundantly as commensals in the skin, mouth, gastrointestinal tract, and genitourinary tract. When pathogenic, they are important causes of skin and soft tissue infection, pharyngitis, pneumonia, bacteremia, and endocarditis. This chapter describes the microbiologic and clinical characteristics of the streptococci typically seen in human infection and emphasizes features of transmission and prevention.