Attosecond extreme ultraviolet and soft X-ray pulses are generated through high-order harmonic generation, a highly non-linear and non-perturbative phenomenon. Nonlinear optical effects and electron-electron interaction play an essential role in various applications of attosecond pulses. The best way to study atomic dynamics under such circumstances theoretically is direct numerical simulation of the time-dependent Schrödinger equation (TDSE). The present article introduces how to model the high-field and ultrafast phenomena on the basis of TDSE, and reviews recent theoretical findings on attosecond pulse generation and attosecond phenomena, peculiar to the time scale of electronic motion inside an atom.