Foam is not just decoration. Done well, it adds aroma at the rim, changes mouthfeel, and can rebalance a drink by softening sharp edges or carrying a top-note flavor into every sip. Done poorly, it collapses fast, tastes “eggy” or bitter, and makes service inconsistent. This guide explains what cocktail foam is, why it forms, the main foam styles used in bars, and how to make foam reliable in real service. It is technique-only and focuses on stability, workflow, and troubles