Context .- Neuroendocrine tumors (NETs) of the gastrointestinal tract have been recognized for more than a century. Despite histologic similarities between different sites in the tract, behavior varies between areas. All of these tumors have malignant potential, but determination of exact risk is difficult. Objectives .- To review the diagnosis of luminal gastrointestinal NETs, including a discussion of grading. Grading by mitotic index/activity, in conjunction with tumor size/stage, has been found to be the strongest predictor of behavior. Data Sources .- Literature review of luminal gastrointestinal NETs was performed and the results summarized. Conclusions .- Our understanding of these lesions is incomplete and continues to evolve.