Serum protein electrophoresis (SPE), serum immunofixation (sIFE) and serum free light chain (sFLC) analysis are all required to monitor intact immunoglobulin multiple myeloma (IIMM) patients. These tests form the basis of response criteria that are discussed below . Immunoglobulin heavy/light chain analysis (Hevylite, HLC) may overcome many of the limitations of SPE (including co-migration and dye saturation issues, Section 17.4), and can provide clarity for patients with ambiguous electrophoresis results. HLC ratios are more sensitive than SPE and sIFE in some patients, and may improve the detection of residual disease and/or disease relapse. In this chapter the roles for sFLC and HLC assays in the routine monitoring of IIMM patients are discussed.