In this chapter optical and electrical properties of quantum-dot mode-locked semiconductor lasers as well as applications based on these devices are discussed. Section 4.1 gives a short overview of different pulse generation and mode-locking techniques, with the main focus on passive mode locking, as well as details on the laser design and advanced features of quantum-dot devices. Timing-jitter reduction and frequency-tuning techniques (hybrid mode locking, optical injection and optical self-feedback) are compared in Sect. 4.2. Section 4.3 is devoted to applications of mode-locked lasers in photonic terahertz signal generation and optical data communication systems.