MVC Question: Should I put form validation rules in the controller or model?

Question

On one hand form validation could be seen as part of the application logic and therefore belonging in the model.

On the other hand, it deals directly with the input

Answer 1

There is another angle to this not covered in the other answers. It depends on what you are saying you Controller / View is! If it is Javascript that checks for validation as users type, for security reasons you should have a validation in your backend as well (this could again be in the controller of your backend or model because anyone can just push Data through Ajax without the browser.

For performance reasons, you should have a validation in your front end controller / view as well because you don't want to be hitting your database every time a user selects an invalid Birth Date or something.

So apart from the theoretical foundation of validation in M, V, and / or C you must also consider the practicality of frontend vs backend irrespective of MVC.

My personal recommendation is to not limit yourself to only one level of validation. An ill placed validation (like the Confirm Password example mentioned in the other answers) can have serious implications on the architecture.