**1. Introduction**

Clonal plasma cell disorders (PCD) including mostly monoclonal gammopathy of undeter‐ mined significance (MGUS) and multiple myeloma (MM) are characterised by expansion of abnormal (clonal) plasma cells (PCs) producing monoclonal protein (M-protein, MIG). Although multiparametric flow cytometry (MFC) allows identification and characterisation of these neoplastic PCs, this approach is used in routine diagnostics of monoclonal gammopathies (MGs) complementarily, mostly in unusual cases [4-6]. The technological development of flow cytometry (FC) in connection with new findings reveal the need for MFC in clinical analysis of MGs. The main applications of immunophenotypisation in MGs are (1) differential diag‐ nosis, (2) determining the risk of progression in MGUS and asymptomatic MM (aMM), (3) detection of minimal residual disease in treated patients with MM, and (4) analysis of prog‐ nostic and/or predictive markers. MFC is also very useful also for research analyses focused on different aspects of B and plasma cell (PC) pathophysiology in term of MG development as well as in looking for potential myeloma-initiating cells. MFC thus should be included as a routine assay in monoclonal gammopathy patients. Clinical significance, usefulness and examples of MFC analyses in MGs are reviewed in this chapter.
