The article reviews and speculates on potential mechanisms underlying sex-related differences in admission patterns, care delivery and outcome of critical illness.Evidence from many countries suggests men are more commonly admitted to intensive care units than are women, and may be more likely to receive aggressive life support. These differences may be confounded by differences in incidence of conditions leading to critical illness, such as acute lung injury and sepsis, both more common among men, or to differences in provision of medical or surgical care that require intensive care unit. There may be different decision-making by patients or decision makers that is dependent upon age and sex of the patient and relation to the surrogate. It is unclear whether differences exist in clinical outcomes; if they do, the magnitude may be greatest among older patients. We describe potential biologic rationales and review animal models. Finally, we explore sex-based differences in the inclusion of men and women in clinical research that underlie our understanding of critical illness.Sex differences in incidence of critical illness and provision of care exist but it is unclear whether they relate to differences in risk factors, or differences in decision-making among patients, surrogates or healthcare professionals.