The new phenomena induced by femtosecond lasers lead to the new area of ultrafast science. It is a significant challenge to explain the phenomena associated with complex non-equilibrium and non-linear processes. Although there is a growing body of experimental observation, a comprehensive model remains undeveloped. We review the challenges in understanding the photon absorption stage mainly for the femtosecond ablation of wide bandgap materials at the intensities of 1013～1014 W/cm2. Major opinions and challenges in ionization mechanisms are presented by primarily considering multiphoton ionization and avalanche ionization.