Core physics and reactor dynamics of high temperature reactors (HTR) are well understood in their basic principles due to the high standards of models and computer code systems available. In what follows an overview is given concerning the basic properties of HTR core design and dynamics code systems. The validation of these systems is accomplished using critical experiments and power plant operation experience. The final verification of the HTR-inherent safety properties calls for further development of the dynamic models, including hypothetical accidental situations such as the ingress of very large amounts of water or air into the primary circuit combined with the total failure of all control systems.