Monoclonal Gammopathy of Clinical Significance (MGCS) is a new umbrella term for all forms of damage to the organism caused by monoclonal immunoglobulin, which is produced by a small non-malignant cell clone, usually in low concentration. However, this monoclonal immunoglobulin has the potential to cause a very diverse range of damage to the organism. ***** The monograph provides comprehensive information about these entities to aid in the early recognition of these very rare diseases. Targeted treatment aimed at the clone producing this monoclonal immunoglobulin has proven effective for treating these forms of damage to the organism, the same treatment used for multiple myeloma or Waldenstrm’s macroglobulinemia. ***** The publication is intended primarily for hematologists, oncologists, and internists.