This is a histological classification of tumours and tumour-like lesions of the exocrine pancreas which also includes those tu­ mours showing a mixture of exocrine and endocrine elements. The classification is based principally on standard microscopic observations, but whenever indicated it incorporates diagnosti­ cally valuable immunohistological findings. In addition, the most important immunohistological findings which are helpful in cat­ egorizing pancreatic tumours are summarised in Table 1. The major guideline of this classification scheme is the group­ ing of the pancreatic exocrine tumours according to their biologi­ cal behaviour. Thus, the neoplasms are broadly divided into benign (adenoma) and malignant tumours (carcinoma). How­ ever, in recent years we have learned that this division is not a sharp but rather a gradual transition. We therefore added a third group which we call "tumours of uncertain malignant potential" representing a borderline category analogous to that recognized for some ovarian tumours. This group includes mucinous cystic tumour, intraductal papillary-mucinous tumour and solid­ pseudopapillary tumour. These neoplasms are defined by the grade of dysplasia and/or potential to become malignant. Mucinous cystic tumours of uncertain malignant potential, for instance, exhibit moderate epithelial dysplasia, but do not show severe dysplasia/carcinoma in situ changes, nor carcinomatous invasion of the cyst wall or the adjacent pancreatic tissue. Solid­ pseudopapillary tumour has a benign looking histological ap­ pearance, but metastases may occur. Biologically, all these neoplasms are primarily slow-growing lesions and have an excel­ lent prognosis when adequately treated by complete resection.