The sepsis concept implies a clinical suspicion of infection and a diagnosis of host's response to this infection. In increasing order of severity are simple sepsis, severe sepsis and septic shock. The semeiology of sepsis consists of systemic signs and elements of information on various visceral dysfunctions. The aetiological investigation is based on the patient's diathesis, on whether the infection is hospital- or community-acquired, on the severity of the sepsis and on simple clinical and paraclinical data. Gram-positive infections are usually caused by pneumococci and staphylococci, and Gram-negative infections by meningococci and enterobacteriaceae. Hospital-acquired infections are remarkable for their frequency, their severity and the diagnostic problems they raise due to patients' colonization and multiple portals of entry. With the exception of some microbiological situations (parasites, fungi, viruses), the main problems raised by non-bacterial sepsis relate to occult sepsis and systemic inflammatory responses to some acute pathologies, such as burns, pancreatitis or multiple injuries. To conclude, redefining the nosology of severe infections to include the fundamental concept of host's response should result in a better evaluation of the infection, its consequences and the treatment of both.