The disclosure features amino lipids and compositions involving the same. Nanoparticle compositions include an amino lipid as well as additional lipids such as phospholipids, structural lipids, PEG lipids, or a combination thereof. Nanoparticle compositions further including therapeutic and/or prophylactic agents such as RNA are useful in the delivery of therapeutic and/or prophylactic agents to mammalian cells or organs to, for example, regulate polypeptide, protein, or gene expression.