Ampulla of Vater (AoV) is a small dilated duct less than 1.5 cm long, formed by the union of pancreatic duct and common bile duct. AoV has also anatomic layer of mucosa, sphincter of Oddi, perisphincteric or duodenal submucosa, and duodenal proper muscle, which corresponds to mucosa, muscularis mucosa, submucosa, and proper muscle layer of other gastrointestinal tract organs, respectively. Because of its small compact size and variation of anatomic structure, it is sometimes difficult to identify layering architecture of AoV. This anatomic difficulty may cause some problem in T classification of ampullary carcinoma (AC). The most confusing point in T classification is the vague definition of T2, “Tumor invades duodenal wall”. It seems that duodenal wall includes duodenal mucosa, submucosa, and proper muscle layer. However there is no precise description or definition about duodenal wall that might lead personal variation in T classification of AC staging. We found that clinical course of AC with perisphincteric and/or duodenal submucosal invasion is more close to AC with T2 than T1. Although it is described as T1b according to T classification scheme of ordinary gastrointestinal tract cancer, we thought AC with T1b may have more high-grade malignant potential than those of other gastrointestinal (GI) tract malignancy. AC showed various clinicopatholgic findings that represent heterogeneous tumor groups within category of AC. Recently site-specific classification of AC was introduced, and it showed relatively well-categorized clinical prognosis. It may be reasonable to understand site-specific tumorigenesis in AC. The standard gross protocol is needed to evaluate pathologic T classification of AC. In conclusion, ampullary neoplasm is composed of various subtypes, which require a separate approach according to anatomic epicenter of ampullary neoplasm. Although submucosal invasion in AC was classified into pT1b, its’ biologic behavior is more close to pT2.