11 © Springer Nature Switzerland AG 2020 J. Moliterno, A. Omuro (eds.), Meningiomas, https://doi.org/10.1007/978-3-030-59558-6_2 D. McGuone Department of Pathology, Yale New Haven Hospital, New Haven, CT, USA e-mail: declan.mcguone@yale.edu A. Huttner (*) Department of Pathology, Yale School of Medicine, New Haven, CT, USA e-mail: anita.huttner@yale.edu 2 Histopathology and Grading of Meningiomas Declan McGuone and Anita Huttner Meningiomas are one of the most common intracranial tumor types encountered by neuropathologists in routine surgical pathology practice. When neuropathologists receive a tissue biopsy from a patient with a meningioma, they typically follow the mandate of the World Health Organization (WHO) Classifcation of Tumors of the Central Nervous System (2016), to accurately classify and grade the tumor. Although meningiomas are usually benign and are often slow-growing tumors, they are notable for their striking histologic diversity, and many different micro- scopic subtypes have been described over the years. Relatively few of these distinct histologic patterns are clinically signifcant, and, in practice, the most commonly encountered subtypes are the meningothelial, fbrous, and transitional variants. In this chapter we will consider the fundamental principles of tumor grading as they apply to meningioma, discuss the major morphologic subtypes of meningioma cur- rently recognized by the WHO, and review common immunohistochemical studies that may be utilized to facilitate a diagnosis of meningioma. The tremendous his- tologic diversity of meningiomas means that they occasionally mimic other tumor types, including several malignant tumors, and this can be diagnostically problem- atic in centers that lack a dedicated neuropathologist. In this chapter we will also consider some of the major differential diagnoses that occasionally masquerade as meningioma.