Interactive Virtual Reality applications are in general complex and non-intuitive. One fundamental problem is that manipulation of 3D objects is very difficult for non-experienced users. We describe a constraint based 3D scene construction system that exploits human intuitions to restrict object placement and interactions. In particular, we focus on the constraints themselves by describing how they are defined, and how they are used within a scene. Several different types of constraints are discussed, including virtual constraints, which decouple the constraint areas from the geometry of an object, and negative constraints, which restrict object placements within certain volumes of space. Furthermore, we discuss techniques that can be used to automatically generate constraints for most geometric objects, which makes incorporating new objects into the system much easier. Finally, we argue that the presented constraint techniques can be incorporated into existing Virtual Reality systems to make interactions easier.