Electron Spin Resonance (ESR) is a powerful analytical method to detect, analyze and determine the characteristics of unpaired electrons in a substance. It is clear that the state of electrons in a substance have a strong influence on its characteristics and functionality, so evaluation by ESR is becoming more and more important. Many types of substances, from electronic materials to catalysts, biological samples, can be studied regardless of whether they are solid, liquid or gas. A wide range of ESR techniques are possible using suitable attachments together with the basic instrument.