> For the complete documentation index, see [llms.txt](https://magdalenaolak.gitbook.io/istqb-foundation-level/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://magdalenaolak.gitbook.io/istqb-foundation-level/2-testing-throughout-the-software-development-lifecycle/2.3.-test-types/2.3.1.-functional-testing.md).

# 2.3.1. Functional Testing

Functional testing of a system involves tests that evaluate functions that the system should perform. Functional requirements may be described in work products such as business requirements specifications, epics, user stories, use cases, or functional specifications, or they may be undocumented. The functions are “what” the system should do. 

Functional tests should be performed at all test levels (e.g., tests for components may be based on a component specification), though the focus is different at each level (see section 2.2). 

Functional testing considers the behavior of the software, so black-box techniques may be used to derive test conditions and test cases for the functionality of the component or system (see section 4.2). 

The thoroughness of functional testing can be measured through functional coverage. Functional coverage is the extent to which some functionality has been exercised by tests, and is expressed as a percentage of the type(s) of element being covered. For example, using traceability between tests and functional requirements, the percentage of these requirements which are addressed by testing can be calculated, potentially identifying coverage gaps.

Functional test design and execution may involve special skills or knowledge, such as knowledge of the particular business problem the software solves (e.g., geological modelling software for the oil and gas industries).


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