Research needs assessment regarding environmental health and safety (EHS) of nanoparticles is problematic. Generating benchmark data to assess research and policy initiatives seems daunting. This study's findings present more granular and qualitative assessments of expert preferences and concerns. This three-round Delphi study elicits expert estimations of problematic nanoparticle characteristics and classifications from a sample of nanoscience experts in chemistry, EHS policy, engineering, environmental toxicology, and human toxicology (n = 18). The Delphi method is a forecasting tool designed for expert evaluation of events under high degrees of uncertainty. Results demonstrate high concordance indicating favorable consensus among the sample concerning characteristics and classifications of nanoparticles that are potentially or actually problematic to EHS. These findings establish a benchmark for future investigations of expert preferences and concerns.