Fusion energy has been actively researched for the last fifty years. Although, there are still many physical and engineering problems to solve, the international scientific community steadily moves forward to that very sacred goal - creation of controlled nuclear fusion and its use as an ultimate energy source for the future. This book includes a detailed description of an optical spectroscopic technique used for critical measurements of fusion plasmas and of the analysis methods. A charge exchange recombination spectroscopy diagnostic is implemented on Alcator C-Mod tokamak to study the high density plasma in a high magnetic field. Density, temperature, and flow velocity of the emitting plasma ions are derived from the analysis of the emission spectra. The application of the CXRS-measured temporally and spatially resolved plasma parameters to the study of the impurity anomalous transport and the ITB (impurity transport barrier) turbulence suppression mechanism was demonstrated.