Parallel Testing is performed when the organization is moving from one system to another.  It is the process of performing workflows in the legacy system and the new system to assure that the processes will lead to the same result.   Parallel testing is also an excellent opportunity to engage more end users.  It allows the user to perform a well-known procedure and then immediately perform the same procedure in the new application.  It highlights the situations where the workflow is changing from the legacy system and creates a list of training points that might be needed.

Some projects only perform parallel testing from one computer application to another computer application.  Some organizations will perform parallel testing from a paper process to the automated process.  It is the type of testing that is most often skipped over but good parallel testing is a valuable method of setting up smooth go-live conversions.

As in unit and integrated testing, select a variety of screnarios.  Often the same scenarios used for integrated testing can be re-purposed.  Test each step against the expected result, document as you go and make training notes.