Deficient or defective coagulation factor VIII (FVIII) and von Willebrand factor (VWF) can cause bleeding through congenital deficiency or acquired inhibitory antibodies. Recent studies on type 1 von Willebrand's disease (VWD), the most common form of the disease, have begun to explain its pathogenesis. Missense mutations of varying penetrance throughout VWF are the predominant mutation type. Other mutation types also contribute while about one-third of patients have no mutation identified. Enhanced clearance and intracellular retention contribute to pathogenic mechanisms. Chromogenic substrate (CS) methods to determine FVIII coagulant activity have several advantages over one-stage methods, which include minimal influence by variable levels of plasma components, notably lupus anticoagulant. Direct proportionality between FVIII activity and FXa generation results in high resolution at all FVIII levels, rendering the CS method suitable for measuring both high and low levels of FVIII activity. FVIII inhibitors in patients with inherited or acquired haemophilia A present several challenges in their detection and accurate quantification. The Nijmegen method, a modification of the Bethesda assay is recommended for inhibitor analysis by the International Society on Thrombosis and Haemostasis. Understanding potential confounding factors including heparin and residual FVIII in test plasma, plus optimal standardization can reduce assay coefficient of variation to 10-20%.These areas are all explored within this article.