*Description* Updatable folds are a notion connected to FunctionalTransformationSystems. *Explanation* The bottom-up traversal scheme associated to a multi-sorted signature, or a system of mutually dependent datatypes, is captured by a family of _fold functions_. These fold functions are parameterized with a _fold algebra_, which stores the functions to be applied at each node during traversal. A fold algebra can be constructed by applying _fold algebra updates_ to a _basic fold algebra_. These notions were coined in DealingWithLargeBananas, and they were used in the FunctionalTransformationSystemsPaper and in StrategiesAsUpdatableFoldAlgebras. *Examples* For a concise example of updatable folds and their application to program transformation, see the FunctionalTransformationSystemsPaper. *See also* FunctionalTransformationSystems