This is Episode 38: Sinonasal I Anatomy, Histology Diversity, and Multimodality Logic. We are now entering a block of head and neck oncology that is genuinely unique. Sinonasal cancers represent fewer than 2,000 cases per year in the United States — less than 1% of all malignancies — yet they pack an outsized punch on written and oral board examinations. Why? Because these tumors test everything at once. You need to know complex three-dimensional anatomy at the interface of the skull base, orbit, and brain. You need to navigate a histologic landscape unlike any other site in the head and neck, where SCC predominates, but a long tail of histologically distinct tumors — esthesioneuroblastoma, adenoid cystic carcinoma, SNUC, mucosal melanoma, and others — each demand fundamentally different management algorithms. And you need to understand why multimodality therapy is the default — not because of randomized trials, which essentially do not exist — but because the retrospective evidence and the anatomic realities of this site demand it. This episode is your anatomic and conceptual foundation. We will map the sinuses and nasal cavity in board-relevant detail, walk through the histology types you must recognize on sight, learn two separate staging systems, and understand the evidence behind the multimodality paradigm. In subsequent episodes, we will tackle site-specific and histology-specific management, planning around the orbit and skull base, and esthesioneuroblastoma as its own entity.