Soon after high-order harmonic generation was discovered in late 1980s, an argument began that the harmonics in the plateau region could be an attosecond pulse train in the time domain. However, it was not until 2001 that the confirmation of this hypothesis was demonstrated. Although the generation of the attosecond pulse train is straightforward, the determination of the number of pulses, the pulse duration, and the phase is rather difficult. In this chapter, we focus on methods for characterizing attosecond pulse trains.