The WHO 2010 grading of GEP NENs used the G3 grade for neuroendocrine neoplasms (NEN) which have a Ki67 index of greater than 20%. It was assumed in the grading system that these neoplasms were poorly differentiated small cell or large cell neuroendocrine carcinomas (NEC). Tumours with well differentiated morphology were referred to as neuroendocrine tumours (NETs) which were further sub-graded as G1 or G2 based on their Ki67 labelling indices being 2 or<2 or 3-20 resp. It became apparent over the ensuing years that there were tumours which by virtue of their Ki67 labelling indices were NECs, but were not poorly differentiated and did not have the very poor outcomes associated with NECs. In the context of the GEP system, the pancreas was often the site of this anomalous situation. It was also apparent that lung tumour pathologists had been referring to G1 and G2 NETs as carcinoids, while using the term atypical carcinoid for a group which were morphologically close to carcinoids but had a higher mitotic rate. The carcinoid group was distinct from small cell or large cell neuroendocrine carcinomas. Meanwhile, data from molecular analysis of NENs leant support to the growing realization that NETs and true NECs should not be separated by proliferation criteria alone. They were molecularly and biologically separate entities, and that NET G3 exists as a well differentiated NEN with a higher proliferation index than NET G2. The new clinico-pathological insights that have led to the current understanding of NEN G3 will be highlighted in the talk.