For the description of mechanisms inducing the damage effects, it is crucial to know interactions and energy deposition of the radiation; risks from ionizing radiation depend both on the radiation quantity and quality. Current concept of the radiation protection is based on the connection between radiation quality and linear energy transfer (LET). One of the methods of determination of LET is based on the chemically etched track detectors. The track detectors are sensitive to heavy charged particles; compared with other methods, they have some advantages (small dimensions and weight, possibility to measure in pulsed beams, in long duration experiments, etc.). This book provides description of methodology of track detectors used for LET spectrometry (at NPI AS CR) and some examples of its use. The detectors were used to determine dosimetric characteristics onboard of aircraft and spacecraft, in some radiotherapeutic beams, or to study fragmentation processes in heavy ion beams. The book could be useful for anyone who is interested in applications where heavy charged particles, both primary and secondary, are needed to be measured, or for those who are working with track detectors.