Abstract: Ionization is a fundamental process during the interaction of matter with an ultrafast intense laser field. The ionization dynamics of atoms and molecules driven by an intense field proceed on a sub-laser-cycle time scale and represent a prototype in exploring attosecond electron dynamics. Moreover, the ultrashort electron bunches produced by the intense laser field can be exploited for measurements of atomic and molecular structure and their ultrafast dynamics. In this review we first briefly introduce the fundamental physics behind intense field ionization; next we discuss a few examples of ultrafast measurements based on the intense field ionized electrons; finally we discuss the prospects of research on intense field ionization.