Plastics contain various chemical substances, which can impact human and ecosystem health and the transition to a circular economy. Meanwhile, information on the presence of individual substances in plastics is generally not made publicly available, but relies on extensive analytical efforts. Here, we review measurement studies of chemicals in plastics and compile them into a new LitChemPlast database. Over 3500 substances, stemming from all plastic life-cycle stages, have been detected in different plastics in 372 studies. Approximately 75% of them have only been detected in nontargeted workflows, while targeted analyses have focused on limited well-known substances, particularly metal(loid)s, brominated flame retardants, and ortho-phthalates. Some product categories have rarely been studied despite economic importance, e.g., consumer and industrial packaging (other than food packaging), building and construction, and automotive plastics. Likewise, limited studies have investigated recycled plastics, while existing measurements of recycled plastics show higher detection frequencies and median concentrations of regulated brominated flame retardants across many product categories. The LitChemPlast database may be further developed or utilized, e.g., for exposure assessment or substance flow analysis. Nonetheless, the plethora of relevant substances and products underscores the necessity for additional measures to enable the transition to a safe circular plastics economy.