In the gastrointestinal (GI) glandular mucosa, the term invasion is ill defined until today. In order to standardize the nomenclature of the expansion of benign and early malignant tumors of the glandular epithelium, we tried to establish a new descriptive terminology: In adenomas, the first glands with neoplastic epithelium may be divided by gland fission before there is primary superficial lateral expansion, followed by luminal expansion in form of budding, glandular fission and minimal branching, and a secondary downward vertical intra- and intertubular expansion into the mucosa. The different growth types of GI adenomas differ in the proportion of these components which will be considered below. In contrast, mucosal carcinomas usually do not show superficial lateral expansion, but they are usually characterized by a primary midmucosal disorderly lateral intertubular expansion with irregular branching and fusion of neoplastic glands, with undermining of the surface epithelium, and with compression and destruction of neighbouring glands by lateral and downward expansion of the neoplastic epithelium, finally with secondary erosions. This pattern of lateral intertubular expansion is indicative for carcinomatous invasion in GI mucosal carcinomas. The permeation of basal membranes usually does not occur in well differentiated carcinomas, which form new basal membranes. With increasing loss of differentiation, there is tumor cell dissociation with permeation or no production of basal membranes up to the pure diffuse type carcinoma; this process of invasion across basal membranes is merely one, but not the only sign of invasion, in our view. A fibrous stromal reaction usually does not occur in the mucosa, but it is most intense in the submucosa and subserosa, and much less pronounced in the muscularis propria. This fibrous stromal reaction is also indicative for invasion. However, Japanese pathologists do not diagnose malignancy only from architectural signs of invasion but mainly from cytological criteria, which also have to be considered aside from structural criteria given above. In comparison, epithelial growth in metaplastic processes is restricted to intratubular expansion or glandular fission without superficial lateral expansion. Pseudoinvasions of cohesive and diffuse types have to be considered for differential diagnosis.