An attacker model is proposed for Cyber Physical Systems (CPS). The attack models derived from the attacker model are used to generate parameterized attack procedures and functions that target a specific CPS. The proposed models capture both physical and cyber attacks and unify a number of existing attack models into a common framework useful for researchers in the experimental assessment of attack detection techniques. The generality of the models is shown by mapping a broad variety of existing attack models to the models proposed here, as well as generating attacks that are not found in the CPS design literature. The models have been used extensively in understanding the impact of cyber attacks on a water treatment system and in the design and assessment of detection mechanisms.