MRI relies on detecting signals in the radiofrequency range that are related to very small energy transitions of the detected molecules. While this is a blessing with regard to the harmless character of MRI, it imposes a serious problem in terms of the low sensitivity caused by almost vanishing spin polarization at ambient temperature. Increasing the sensitivity through special preparation of the spin system prior to the encoding and detection is therefore a powerful approach. The achieved hyperpolarization has enabled various applications for molecular and cellular imaging. This tutorial will summarize aspects of polarization methods, probe design and signal encoding.