Increasing use in everyday life, plastic owes to its good functional properties, easy processing and low production costs. In addition to a number of good properties, large amounts of plastic pose significant challenges as they end up neglected in the environment, where the aquatic environment is particularly endangered. Exceptional resistance to external conditions, complex decomposition and durability do not alleviate the situation. Many animals replace food with plastic waste, so plastic enters the food chain and thus directly threatens human health. The situation is further complicated by various additives that are added for the purpose of modifying the properties and easier processing. Plasticizers, fillers, stabilizers, flame retardants and dyes are just some of the examples of the main groups of additives. Although most of them are necessary and extremely useful for the application and development of plastics, there are also toxic additives that have been regularly added to plastics until a few years ago. Today, their use and quantity is limited by special regulations and conventions, and they are classified into groups such as endocrine–disrupting–chemicals (EDCs) or persistent organic pollutants (POPs). Ecotoxicological studies are important for full understanding and awareness of the impact of additives and plastics on the environment and organisms. Animals are common laboratory models in such research, but nowdays they are increasingly being replaced by mathematical and simulation models. This study will provide an overview of the types of plastics and additives, and previous ecotoxicological studies conducted on additives of polybrominated diphenyl ethers (PBDEs) and heavy metals.