Neuroendocrine neoplasms (NENs) are a family of individually unique tumors with prognosis that ranges from weeks to decades. Clinicians struggle to provide a prognosis for individual patients, and there is a genuine, unmet, need for accurate prognostic biomarkers. This chapter describes current and novel prognostic markers, with a focus on gastroenteropancreatic (GEP) NENs, and the biological attributes of each biomarker. Traditional clinical markers of primary site, stage, and grade that are inherently prognostic by design are discussed. Blood-based biomarkers that could be more amenable to patient monitoring are also explored. In addition, more recent studies describing molecular features of GEP-NENs including gene mutations, chromosome aneuploidy, and gene expression changes are discussed and their potential prognostic value examined. Finally, functional imaging studies that define tumor somatostatin receptor expression with respect to metabolic activity that are emerging prognostic biomarkers are also examined.