Abstract : This paper reviews the finite-difference time-domain (FDTD) model emerging since 2002 to analyze and design traveling-wave photodetectors (TWPDs). The TWPD is a key microwave-photonics component to convert optical signal to electrical current, and this physical conversion is considered in the full-wave numerical model with allowance of calculating the impulse response of the device. This numerical model afford important device designs for large bandwidth by considering geometrical factors, carrier transit property, impedance mismatch, and low-temperature growing effect.