Feature modeling is nowadays the predominant way of product modeling. Functional and geometric information are stored in a single product model, consisting of features such as holes, slots, and ribs. An emerging technology is the application of concepts of feature modeling to freeform shapes. Of particular importance here is that there are often many validity conditions, or constraints, that the resulting freeform feature models should satisfy. An example of this is a maximum curvature constraint on the faces of a particular feature. This paper presents an approach to specify and maintain such validity conditions for freeform feature models.