Published experimental photon and electron impact ionization cross sections of CH4 have been reviewed and analyzed. Absolute partial ionization oscillator strengths (fij) for CH4+, CH3+, CH2+, CH+, H+, and H2+ have been obtained. Electron impact ionization cross sections are recommended based on agreement between the oscillator strength values derived from photoionization and electron impact measurements. Analytic functions for cross sections of CH4+, CH3+, CH2+, CH+, C+, H2+, and H+ produced by electron impact ionization of CH4 are established. The derived excitation cross-section functions are accurate from threshold to impact energies limited by relativistic effects (i.e., E < 0.2 MeV). The cross sections examined here are important for modeling Titan ionospheric chemistry.