Due to the inherent timing jitter of the electronic clocking circuits and comparator ambiguity, the traditional electronic analog-to-digital conversion (ADC) cannot fulfill the development of high bandwidth digital signal processing. Analog-to-digital conversion consists of sampling, quantizing and coding. Introducing photonic technologies for sampling and quantization of the electrical analog signal, all-optical ADC can improve the performance of the digital signal processing system to achieve high sampling rate and high resolution. Hence, it solves the bottleneck problem of electronic ADC. The main recently developed all-optical analog-to-digital converters, such as those based on Taylor scheme, interferometric and polarization interference, phase-shifted optical quantization (PSOQ), soliton self-frequency shift, long-period waveguide grating (LPWG) and arrayed waveguide grating (AWG), are introduced. Meanwhile, the characteristics of different methods are analyzed.