The main goal of this chapter is to give a brief introduction in magnetic resonance imaging physics. First a historical introduction about the discovery of the nuclear magnetic resonance effect and its way into daily clinical routine is given. Today, MRI is a widely used imaging technology. Besides its noninvasiveness, its humongous ability to generate different contrasts in soft tissues gives rise to the increasing importance in diagnosis of different diseases. In addition to its ability to provide high-resolution anatomical images it is also possible to directly obtain molecular information. To understand the principles of molecular imaging it is important to get a basic overview about magnetic resonance and imaging principles. This overview is followed by a description of main hardware components and safety-related topics and an introduction into possible accessible molecular information.