Streptococcus pyogenes infects the human pharynx, causing pharyngitis and tonsillitis, and as secondary complications, it can lead to acute glomerulonephritis and rheumatic fever. In recent years in Japan, there has been a trend of increasing cases of severe invasive streptococcal infections, which involve soft tissue necrosis, shock, and multiple organ failure. Streptococcus pneumoniae often resides in the nasal cavities of children, causing otitis media and pneumonia. In pneumonia among the elderly, it is frequently detected as a causative agent, and it has long been the case that more than half of the clinical isolates exhibit resistance to antibiotics. This overview discusses the responsible molecules and pathogenic mechanisms involved in these streptococcal infections.