# Expressions Reference Overview

Expressions are written in Telerik Reporting scripting language. The language is based on mathematical expressions and is case-insensitive.

When you write an expression, you have access to many built-in fields, collections, and functions that you can use alone or combine with other terms. You can add function calls, operators, constants, fields, parameters, and items from the built-in collections to an expression. Built-in function calls include aggregation, commonly used utility functions, and conversion methods. You can also add references to functions defined in your code (see [user functions](https://www.telerik.com/products/reporting/documentation/designing-reports/connecting-to-data/expressions/extending-expressions/user-functions)).