In this review, we provide a brief summary of the timing jitter of optical pulse sources. First, we introduce and compare three typical measurement methods of timing jitter: a direct measurement method, a second harmonic cross-correlation method, and a phase noise measurement method. Then the features and the reduction technique of the timing jitter of gain-switched semiconductor lasers are briefly explained. Finally, we propose and demonstrate a simple method for measuring the timing jitter of a distributed feedback (DFB) laser and a Fabry-Perot (FP) laser using a delayed optical feedback system.