The objective of this work has been to test and model the machine properties including the mechanical properties and the electrical properties in resistance welding. The results are used to simulate the welding process more accurately. The state of the art in testing and modeling machine properties in resistance welding has been described based on a comprehensive literature study. The present thesis has been subdivided into two parts: Part I: Mechanical properties of resistance welding machines. Part II: Electrical properties of resistance welding machines. In part I, the electrode force in the squeeze stage has been measured for two machines. It was found that for the same level of electrode force, the pneumatic machine takes longer time to build up and stabilize the force to the static value due to the compressibility of the air. The hydraulic one is very fast to reach and stabilize to the static electrode force, and the time of stabilizing does not depend on the level of the force. An additional spring mounted in the welding head improves the machine touching behavior due to a soft electrode application, but this results in longer time of oscillation of the electrode force, especially when it is lower than the spring force. The work in part I is focused on the dynamic mechanical properties of resistance welding machines. A universal method has been developed to characterize the dynamic mechanical behaviour of C-frame machines. The method is based on a mathematical model, in which three equivalent machine parameters were determined, i.e. equivalent moving mass, equivalent damping coefficient and equivalent spring constant. A specially designed breaking test is applied for determining these three parameters. The method, which is confirmed by a series of “supported breaking tests” as well as real projection welding tests, is easy to realize in industry, since tests may be performed in situ. In part II, an approach of characterizing the electrical properties of AC resistance welding machines is presented, involving testing and mathematical modelling of the weld current, the firing angle and the conduction angle of silicon controlled rectifiers with the aid of a series of proof resistances. The model predicts the weld current and the conduction angle (or heat setting) at each set current, when the workpiece resistance is given. As a part which was not originally planned, a new approach for determining the dynamic resistance in resistance welding by measuring the voltage on the primary side and the current on the secondary side is suggested. This method increases the accuracy of measurement because of higher signal – noise ratio, and allows in-process application without any wires connected to the electrodes. In order to test the reliability of such an approach, the results are compared with those obtained by the conventional method. Furthermore, the proposed method is used to measure the faying surface contact resistance.