Unit vs Integration vs System vs E2E Testing

The table below illustrates the most critical characteristics and differences among Unit, Integration, System, and End-to-End Testing, and when to apply each methodology in a project.

	Unit Test	Integration Test	System Testing	E2E Test
Scope	Modules, APIs	Modules, interfaces	Application, system	All sub-systems, network dependencies, services and databases
Size	Tiny	Small to medium	Large	X-Large
Environment	Development	Integration test	QA test	Production like
Data	Mock data	Test data	Test data	Copy of real production data
System Under Test	Isolated unit test	Interfaces and flow data between the modules	Particular system as a whole	Application flow from start to end
Scenarios	Developer perspectives	Developers and IT Pro tester perspectives	Developer and QA tester perspectives	End-user perspectives
When	After each build	After Unit testing	Before E2E testing and after Unit and Integration testing	After System testing
Automated or Manual	Automated	Manual or automated	Manual or automated	Manual