The paper gives an overview of state of the art models and techniques for simulation of thermal effects in RTP, RTCVD and CVD reactors. The methods are exemplified with recent applications. The examples include the investigation of susceptively-heated single wafer reactors including effects related to optical properties of the wafer. Natural convection and wafer rotation are discussed in a further example. The steady state and transient uniformity control in a RTP system is treated together with a method to supplement simulation with experimental data. As a recent topic the thermal and gravitational stress in 300 mm wafer is discussed.