Although factor XI (FXI) deficiency has a particularly high incidence in Ashkenazi Jews, it is now frequently diagnosed in other ethnic groups. This review gives an overview of the basic pathophysiology, clinical manifestations, and management of FXI deficiency. The correlation between FXI levels and the bleeding phenotype is much less clear than in the haemophilias, and consequently the bleeding risk can be difficult to predict. Two well-characterized mutations in the F11 gene are responsible for the majority of Jewish cases, but new mutations are becoming increasingly recognized. The publication of the crystal structure has greatly enhanced our understanding of the structure-function relationship in FXI. The impact of recent studies on our understanding of the role of FXI in coagulation is discussed.