We know how to customize and extend Dynamics 365 CE, so let's talk about testing our customization and developments. Depending on the methodology you are using, you may need to perform testing when the development stage is over, such as in the waterfall model or when you need to perform testing when the first sprint is over, for example, in the agile model. Whatever approach is selected, testing for our project is performed based on the test plan, so it is critical to have a plan in place before the testing phase is started. Before preparing a test plan, we need to understand what a test plan is.