In this Spotlight, we focus on silicon-based nonlinear optical signal processing and review the recent research progress of nonlinear optical signal processing for handling high speed digital optical signals on silicon photonics platforms. A summary of previously demonstrated typical silicon-based nonlinear optical signal processing functionalities since 2004 is shown in Table 1. The most widely used devices are nonlinear waveguides and MRRs. Photonic-crystal waveguides are also used for nonlinear optical signal processing because of how their nonlinearities enhance slow light effects. A photonic-crystal nanocavity is also a good candidate due to its strong nonlinearities induced by ultra-small mode volume. Another important characteristic is the modulation format. The modulation formats used in early demonstrations are almost on–off keying or differential phase-shift keying formats. Beyond binary modulation formats, advanced multilevel modulation formats together with multiplexing techniques - for example, quadrature phase-shift keying (QPSK), m-ary phase-shift keying (m-PSK), m-ary quadrature amplitude modulation (m-QAM), and orthogonal frequency division multiplexing (OFDM) - have been widely used in optical fiber communication systems to increase the transmission capacity and spectral efficiency. Recently, some silicon-based nonlinear optical signal processing functionalities that employ advanced multilevel modulation formats have been demonstrated.