Membranous nephropathy (MN) most commonly is idiopathic, but secondary causes are common in children and in older adults. The most common secondary causes of MN in industrialized countries include malignancy and systemic lupus erythematosis. Infectious causes (hepatitis B, quartan malaria, schistosomiasis) remain the most common etiologies in endemic areas. In this article we describe the clinical approach to patients with MN associated with malignancy and other common secondary causes. Treatment of secondary MN generally targets the primary disease rather than the renal lesion.