AbstractX-ray spectra from Alcator C-Mod plasmas have been collected using a high wavelength resolution, five-spectrometer array during a wide range of operating conditions, providing a large variety of diagnostic information. Each independently scannable von Hamos-type spectrometer has a wavelength range of 2.6–4.1 Å and the complete Rydberg series of He- and H-like argon has been observed. Spectra of Δn = 1 ground state transitions and satellites taken along various chords have been simulated using the results from a collisional radiative model and the MIST transport code. Line ratios are used to infer the electron temperature profile and good agreement is found with electron cyclotron emission profiles. Line intensities have been used to obtain absolute argon densities and argon recycling coefficients, and up-down asymmetric density profiles are sometimes observed. The widths of the strongest lines have been used to deduce ion temperature profiles, and wavelength shifts have been used to determine toroidal rotation velocities. Transitions from around n = 9 to the ground state are populated by charge exchange in the outer regions of the plasma and these line intensities have been used to measure neutral hydrogen density profiles. Spectra from helium-like scandium have been obtained during injection experiments and time histories and line intensities have been used to determine impurity transport coefficients. Δn≥2 ground state transitions in molybdenum charge states around neon-like; have been observed and used to measure the absolute molybdenum density and to test atomic structure calculations.