Definitions have been considered important in all fields of medicine, both at a patient level to facilitate accurate diagnosis and treatment, and at a research level to clarify patient inclusion criteria and interpretation of study results. Although there is agreement that sepsis refers to the host response to infection, the complexity of this response and of the patient groups affected, however, has meant that establishing accepted definitions of sepsis has been difficult. Recent consensus has provided global definitions of sepsis and infection, but further work is necessary to provide a means of more completely characterizing the sepsis response in individual patients, such that new interventions can be targeted better as physicians strive to decrease the still high mortality rates associated with this condition.