D-dimer has become one of the commonly requested coagulation tests, especially in the venous thromboembolism rule out setting. Appropriate D-dimer testing has significantly decreased the number of radiological investigations in this clinical scenario. D-dimer testing also plays an important role in the diagnostic process of the systemic coagulation disorders, especially disseminated intravascular coagulation in conjunction with other coagulation tests. However, widespread D-dimer testing without good understanding of the technical issues related to the test has recently caused some concerns. This review aims to address the basic physiology of D-dimer formation, the different methods available for its routine assessment, the current problems that both clinicians and laboratory scientists face with this test, and the important need for harmonization of commercial immunoassays.