Measurement of the chemical composition of cosmic rays are essential for understanding their origin, transport and interactions in the Galaxy. In particular the Boron to Carbon (B/C) ratio is crucial for modeling the cosmic ray propagation. This book provides a measurement of the B/C ratio in the kinetic energy range 0.35–45 GeV/nucleon. This measurement are based on the data collected by the Alpha Magnetic Spectrometer AMS-01 during the STS-91 space shuttle flight in 1998 June. The role of light nuclei in the cosmic ray propagation physics are reviewed and discussed.