What is Exploratory Testing and when should it be performed?

The definition of Exploratory Testing is “simultaneous test design and execution” against an application. This means that the tester uses her domain knowledge and testing experience to predict where and under what conditions the system might behave unexpectedly. As the tester starts exploring the system, new test design ideas are thought of on the fly and executed against the software under test.

On an exploratory testing session, the tester executes a chain of actions against the system, each action depends on the result of the previous action, hence the outcome of the result of the actions could influence what the tester does next, therefore the test sessions are not identical.

This is in contrast to Scripted Testing where tests are designed beforehand using the requirements or design documents, usually before the system is ready and execute those exact same steps against the system in another time.

Exploratory Testing is usually performed as the product is evolving (agile) or as a final check before the software is released. It is a complimentary activity to automated regression testing.