We report on the development and application of a parametrized crystal-field model for both C1 symmetry centers in trivalent erbium-doped Y2SiO5. High-resolution Zeeman and temperature dependent absorption spectroscopy was performed to acquire the necessary experimental data. The obtained data, in addition to the ground (4 I15/2 Z1) state and exited (4 I13/2 Y1) state Zeeman and hyperfine structure, were simultaneously fitted in order to refine an existing crystal-field interpretation of the Er3+:Y2SiO5 system. We demonstrate that it is possible to account for the electronic, magnetic, and hyperfine structure of the full 4 f 11 configuration of Er3+:Y2SiO5 and further, that it is possible to predict both optical polarization behavior and high magnetic field hyperfine structure of transitions in the 1.5-μm telecommunications band.