**1. Introduction**

Myeloid leukemia regroups a variety of myeloid disorders. Some are more frequent and well characterized such as acute myeloid leukemia (AML) or chronic myeloid leukemia (CML). However, other clonal myeloid disorders, designed myelodysplastic/myeloproliferative neoplasms (MDS/MPN), are still subjected to diagnosis and therapeutic challenges. MDS/MPN might possess both dysplastic and proliferative features and cannot simply be listed in myelodysplastic syndrome (MDS) and chronic myeloproliferative disorder (CMPD) categories. Currently, three distinct groups are well classified: chronic myelomonocytic leukemia (CMML), juvenile myelomonocytic leukemia (JMML), and atypical chronic myeloid leukemia (aCML). An additional group called myelodysplastic/myeloproliferative neoplasm-unclassifiable (MDS/MPN-UC) has also been included. Apparently, MDS/MPN-UC shows both MDS and CMPD characteristics but differs at some extent from the other three MDS/MPN groups. MDS/MPN implicates defects in the modulation of myeloid pathways leading to cell survival and proliferation. However, the etiology of the defects remains elusive.

In this introductory chapter, we have succinctly described each of the myeloid disorders and provided some highlights on diagnosis and available therapies.
