Polymer hydrogels have been used for many years in European and Asian countries, and these products are often considered to be the same material in different packaging. This, however, is not the case. Performance and safety profiles depend on many factors including chemical and physical characteristics (including rheological properties), manufacturing process and control (cross linking, impurities, stability, etc.), injection technique, and interaction with surrounding tissues. Polyacrylamide hydrogel (PAAH) products, although often considered equal, have clear differences in composition, manufacturing, and injection technique as well as ability to interact with surrounding tissues, characteristics that determine the safety and effectiveness profiles of each of these gels.