Molecular imaging is a medical and scientific discipline that enables a perturbation-free visualization of functional molecular processes in living organisms. It differs from the conventional imaging techniques by using probes known as biomarkers or tracers that interact physically or chemically with their surroundings and target certain metabolic processes or abnormalities. In this chapter, we want to present a closer look at today's most used and most promising molecular imaging modalities. We focus on modalities based on magnetic resonance spectroscopy and imaging combined with hyperpolarized probes, radiotracer-based single photon emission computed tomography (SPECT) and positron emission tomography (PET). Finally, emerging optical imaging modalities including bioluminescence imaging (BLI), fluorescence imaging (FI) and multi-spectral optoacoustic tomography (MSOT) are also described.