The chapter provides an overview of the classification of myeloid neoplasms in the 4th edition of the WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues. It emphasizes the integration of clinical, morphological, immunophenotypic, and genetic features to define disease entities. The classification is based on the principles initially defined in the Revised European-American classification of lymphoid neoplasms (REAL) and applies these principles to myeloid and histiocytic neoplasms. The chapter highlights the importance of defining 'real' diseases that can be recognized by pathologists using available techniques and that appear to be distinct clinical entities. It also discusses the recognition that the underlying causes of these neoplasms are often unknown and may vary, and that the classification incorporates all available information, including morphology, immunophenotype, genetic features, and clinical features. The chapter outlines the prerequisites for the classification of myeloid neoplasms by WHO criteria, including the importance of evaluating peripheral blood and bone marrow aspirates, and the need for cytogenetic and molecular genetic studies. It emphasizes the integrated, multimodality approach required for diagnosing and classifying these neoplasms and recommends correlating various diagnostic studies with clinical findings in a single integrated report.The chapter provides an overview of the classification of myeloid neoplasms in the 4th edition of the WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues. It emphasizes the integration of clinical, morphological, immunophenotypic, and genetic features to define disease entities. The classification is based on the principles initially defined in the Revised European-American classification of lymphoid neoplasms (REAL) and applies these principles to myeloid and histiocytic neoplasms. The chapter highlights the importance of defining 'real' diseases that can be recognized by pathologists using available techniques and that appear to be distinct clinical entities. It also discusses the recognition that the underlying causes of these neoplasms are often unknown and may vary, and that the classification incorporates all available information, including morphology, immunophenotype, genetic features, and clinical features. The chapter outlines the prerequisites for the classification of myeloid neoplasms by WHO criteria, including the importance of evaluating peripheral blood and bone marrow aspirates, and the need for cytogenetic and molecular genetic studies. It emphasizes the integrated, multimodality approach required for diagnosing and classifying these neoplasms and recommends correlating various diagnostic studies with clinical findings in a single integrated report.