# How is Expression Transformation different from the Function Transformation?

In the [*Expression*](https://documentation.astera.com/dataflows/transformations/expression-transformation) transformation, you can define an expression either from the library of built-in functions in the expression builder or write your own expression. You can also modify and use variables for various calculations in the expression transformation. Whereas in the [*Function*](https://docs.astera.com/projects/centerprise/en/8/transformations/filter-transformation.html) transformation, you can only work with a set of built-in functions. You cannot modify or customize these built-in functions to suit your requirement, as you can do in the Expression transformation.