Plastic waste is generated at a rate of 400 Mt year⁻¹, with limited understanding of its environmental persistence. This Perspective summarizes degradation rates and pathways for major thermoplastic polymers, introducing the specific surface degradation rate (SSDR) to harmonize measurements and estimate half-lives. SSDR values vary widely, with marine HDPE ranging from 0 to 11 μm year⁻¹. Using a mean SSDR for marine HDPE, estimated half-lives range from 58 years (bottles) to 1200 years (pipes). Surprisingly, HDPE and PLA have similar SSDR in the marine environment, despite PLA degrading 20 times faster on land. The study highlights the need for better experimental studies, standardized reporting, and simulation methods for polymer degradation. Plastics are synthetic polymers made from organic monomers, with the largest market share belonging to low-cost commodity thermoplastics. Most plastics are made from nonrenewable petrochemicals, making them durable and long-lived when discarded. Only 18% of plastics are recycled, with 58% landfilled or entering the environment, where they accumulate for long periods. In the U.S., plastics make up 19% of municipal solid waste, with projections of 12,000 Mt of plastic waste by 2050. Plastic waste entering oceans is a major concern, with large accumulations in the South Pacific and Eastern Pacific gyres. Plastic waste in oceans is expected to grow from 50 Mt in 2015 to 150 Mt by 2025. Most ocean plastic comes from 10 rivers in Asia and Africa. Ocean plastic debris is associated with persistent organic pollutants, and nearly 700 marine species interact with plastics through ingestion, entanglement, or smothering. Degradation rates reported in peer-reviewed literature often differ from those in the popular press, with media often presenting degradation times as known despite limited scientific evidence. Degradation times for plastic bags range from 10–20 years to 500–1000 years, while plastic bottles are reported as over 70 to 450 years. Some media claim plastics do not degrade at all, but the type of plastic and environmental conditions are often unclear. Environmental degradation mechanisms for plastics include physical (e.g., cracking, embrittlement) and chemical (e.g., bond cleavage, oxidation) processes. Chemical degradation involves hydrolysis, oxidation, and microbial action. Abiotic and biotic processes often work together, with abiotic degradation leading to smaller molecules that are mineralized by microbes. Degradation mechanisms for PE, PET, and PLA are discussed, with PET degrading primarily through photo-oxidative and thermal oxidative processes in the marine environment. Assessing plastic degradation involves methods to measurePlastic waste is generated at a rate of 400 Mt year⁻¹, with limited understanding of its environmental persistence. This Perspective summarizes degradation rates and pathways for major thermoplastic polymers, introducing the specific surface degradation rate (SSDR) to harmonize measurements and estimate half-lives. SSDR values vary widely, with marine HDPE ranging from 0 to 11 μm year⁻¹. Using a mean SSDR for marine HDPE, estimated half-lives range from 58 years (bottles) to 1200 years (pipes). Surprisingly, HDPE and PLA have similar SSDR in the marine environment, despite PLA degrading 20 times faster on land. The study highlights the need for better experimental studies, standardized reporting, and simulation methods for polymer degradation. Plastics are synthetic polymers made from organic monomers, with the largest market share belonging to low-cost commodity thermoplastics. Most plastics are made from nonrenewable petrochemicals, making them durable and long-lived when discarded. Only 18% of plastics are recycled, with 58% landfilled or entering the environment, where they accumulate for long periods. In the U.S., plastics make up 19% of municipal solid waste, with projections of 12,000 Mt of plastic waste by 2050. Plastic waste entering oceans is a major concern, with large accumulations in the South Pacific and Eastern Pacific gyres. Plastic waste in oceans is expected to grow from 50 Mt in 2015 to 150 Mt by 2025. Most ocean plastic comes from 10 rivers in Asia and Africa. Ocean plastic debris is associated with persistent organic pollutants, and nearly 700 marine species interact with plastics through ingestion, entanglement, or smothering. Degradation rates reported in peer-reviewed literature often differ from those in the popular press, with media often presenting degradation times as known despite limited scientific evidence. Degradation times for plastic bags range from 10–20 years to 500–1000 years, while plastic bottles are reported as over 70 to 450 years. Some media claim plastics do not degrade at all, but the type of plastic and environmental conditions are often unclear. Environmental degradation mechanisms for plastics include physical (e.g., cracking, embrittlement) and chemical (e.g., bond cleavage, oxidation) processes. Chemical degradation involves hydrolysis, oxidation, and microbial action. Abiotic and biotic processes often work together, with abiotic degradation leading to smaller molecules that are mineralized by microbes. Degradation mechanisms for PE, PET, and PLA are discussed, with PET degrading primarily through photo-oxidative and thermal oxidative processes in the marine environment. Assessing plastic degradation involves methods to measure