This Fascicle on non-neoplastic disorders is devoted to the assessment of bone marrow in pediatric and adult patients, though neoplastic processes are briefly highlighted in discussions of differential diagnostic considerations. Lineage-based non-neoplastic erythroid, myeloid, megakaryocytic, and lymphoid disorders are discussed as well as bone marrow findings in patients with systemic, nonhematologic disorders. A standardized format is used to the best extent possible for each chapter, with an emphasis on categorizing disorders based on patient age, constitutional versus acquired conditions, and pathogenetic mechanism. Because of this combined lineage-based and disease-based approach, there is some unavoidable overlap of content between chapters. Cross referencing refers the reader to the specific chapter with the most comprehensive discussion.