As a major hematopoietic and lymphoid organ, morphological evaluation of the bone marrow is an important component of toxicity or safety assessment studies. While definitive characterization of bone marrow lesions often requires cytological aspirates or smears, assessment of histological bone marrow sections provides information regarding tissue architecture and hematopoietic status that is relevant for the detection of direct or indirect responses to chemical exposure. A variety of lesions have been observed in bone marrow. For example, lesions involving disturbances in growth, degenerative changes, inflammatory changes and neoplasia have been described. Lesions identified in hematoxylin and eosin-stained sections typically represent changes in the hematopoietic cell lineage and/or stromal cells since definitive identification of lymphoid cells is difficult except in cases of lymphoma. This review provides a descriptive and pictorial representation of a wide range of bone marrow lesions. Since large animal-to-animal variation may exist and there can be collection site- and age-related differences, it is imperative that the pathologist reviews all potential treatment-related findings against appropriate concurrent controls.