The book starts with an introduction to and history of myeloid-derived suppressor cells (MDSCs), followed by a description of their differentiation, their role in the tumour microenvironment and their therapeutic targeting. It closes with an outlook on future developments. In cancer patients, myelopoiesis is perturbed and instead of generating immunogenic myeloid cells (such as dendritic cells, inflammatory macrophages and granulocytes), there is an increase in highly immature MDSCs. These cells are distributed systemically, resulting in general immunosuppression. They also infiltrate tumours, promoting their progression and metastasis by inhibiting the natural anti-tumour immune response. As these cells also interact with classical anti-neoplastic treatments, they have become major therapeutic targets in the pharmaceutical industry and in oncology research.