Since the 1980s, there has been a marked increase in the recognition and reporting of highly invasive group A streptococcal (GAS) infections associated with shock and organ failure, with or without necrotizing fasciitis. Such dramatic cases have been defined as streptococcal toxic shock syndrome (StrepTSS). Strains of GAS isolated from patients with invasive disease have been predominantly M types 1 and 3, which produce either pyrogenic exotoxin A or B or both. The clinical and demographic features of streptococcal bacteremia, myositis, and necrotizing fasciitis are presented and compared with those of StrepTSS. Current concepts in the pathogenesis of invasive streptococcal infection will be presented, with emphasis on the interaction between GAS virulence factors and host defense mechanisms. Finally, new concepts in the treatment of StrepTSS will be discussed.