Multiple myeloma is a hematological cancer which is characterized by malign proliferation of plasma cells and their precursors. Although myeloma cells express antigens which can be recognized by host T cells they can be rarely eradicated by the immune system because of the immunsupression by tumor microenvironment. The immunsuppressive regulatory T cells (Treg) with CD4+ CD25+ Foxp3+ fenotype are increased in peripheral blood of multiple myeloma patients which have key roles on oto-immune control, T-cell homeostasis, immunmodulation of all the immune response. This book is a combination of Master of Science and PhD dissertations examining the role of Treg cells in multiple myeloma. The interaction of Treg cells with autologous peripheral blood stem cell transplantation and bortezomib treatment is evaluated by ex vivo and in vitro tests. The findings point out the importance of Treg cells in multiple myeloma and the experimental design will help the researchers interested in the same field.