The term ‘dysplasia’ refers to ‘an unequivocal neoplastic epithelial alteration without invasive growth’. The term ‘intraepithelial neoplasia’ often replaces ‘dysplasia’ in World Health Organization (WHO) guidance. Dysplasia is a precursor lesion of cancer and a marker for high cancer risk, offering a window of opportunity for early detection and cure of neoplasia. Most pathologists now classify columnar dysplasia as low grade (LGD) and high grade (HGD). The criteria for grading dysplasia include both cytological and architectural abnormalities. The diagnosis of dysplasia can be challenging in some clinical settings, especially when there is a background of active or resolving inflammation [e.g., in Barrett’s oesophagus (BO) or inflammatory bowel disease (IBD)] that may cause reactive epithelial atypia. In addition, there is significant inter- and intra-observer variability for the diagnosis and grading of dysplasia. The variability may reflect the limitations of morphology-based criteria and has led to the development of adjunctive diagnostic methods such as immunohistochemistry. These methods, although promising, are controversial and require evaluation in further studies. This chapter describes the classification, microscopic features, and grading of dysplasia at different sites in the gastrointestinal (GI) tract.