A spin label (SL) is an organic molecule which possess an unpaired electron and the ability to bind another molecule. Spin labels are usually used as a tool for probing proteins or biological membranes local dynamics using EPR spectometry. The site-directed spin labeling (SDSL) technique allows to monitor a specific region within the protein. Spin labels have usually an unpaired spin on nitrogen atom. In protein structure examinations aminoacid specific SL can be used.