Electrochemical luminescence (ECL) is the process where species generated at electrodes undergo electron transfer reactions to form excited states that emit light. Application of a voltage to an electrode in the presence of an organic or inorganic luminophore, such as diphenylanthracene (DPA) or Ru(bpy)32+ (where bpy = 2,2′-bipyridine), results in light emission. By employing ECL-active species as labels on biological molecules, ECL has found application in commercial instruments to detect many clinically relevant analytes (e.g., immunoassays and DNA probes) at sub-picomolar concentrations. The principles, ECL emitting systems and applications are outlined in this review with a focus on discoveries made in the past few years.