# Using expressions

Expressions are structured strings that are parsed and evaluated by Talend Cloud API Tester.

You can use expressions in the following places:

* Request URL
* Headers (name and value)
* Query parameters (name and value)
* Form parameters (name and value)
* Request body (of type text)
* Assertions values

They allow you to:

* Reference environment variables by name
* Reference other request elements by name
* Manipulate your data

Expressions can only reference a request element, an environment variable or a method. You cannot reference a whole project, service or scenario.

Talend Cloud API Tester does not support numbers higher than 9,007,199,254,740,991. Higher numbers may be automatically modified.

For example, if a request body contains the number 900719925474099324686842, it appears as follows in the Expression Builder: ![In the Preview field, the number is 9.007199254740993e+23.](https://help.qlik.com/talend/en-US/api-tester-user-guide/Resources/images/expression_builder_big_number.png)

A notification appears when a number is not supported, you can disable these notifications in API Tester settings > General.

### In this section

* [Reusing parts of requests or responses](https://help.qlik.com/talend/en-US/api-tester-user-guide/Cloud/reusing-parts-of-requests-or-responses)
* [Transforming your data](https://help.qlik.com/talend/en-US/api-tester-user-guide/Cloud/transforming-data)
* [Building an expression](https://help.qlik.com/talend/en-US/api-tester-user-guide/Cloud/building-expression)