Adding forms to your application seems to be an easy task, but when you start coding the server-side code, the task of validating user input grows bigger and bigger as the form becomes more complex. Security is paramount, as the data is from an untrustworthy source, and is going to be entered into the database. WTForms is a library that handles server form validation for you by checking input against common form types. Flask WTForms is a Flask extension that is built on top of WTForms that adds features, such as Jinja HTML rendering, and protects you against attacks, such as SQL injection and cross-site request forgery. This extension is already installed in your virtualenv, because it's declared in the requirements.txt file.