Where should validation logic be implemented?

Question

When developing my interfaces (contracts) and the concrete implementations of them, both the data models as well as repositories, I find myself questioning where the validation

Answer 1

Validation rules should be defined at the class level in an abstract fashion that can both 1) be run in the class's native environment 2) be rendered as rules for other dependent environments, such as UI scripting or repository procedures, as needed.

This gets you the logic centralized where it should be, in the class, and ancillary validation in the UI and wherever else -- easily maintainable since it's derived from the class rather than being detached logic living in a disconnected location. All-around win.