Considering the cost and complexity, intensity-modulation and direct-detection (IM-DD) have played significant roles in short-distance optical networks, including optical access networks, intra- and inter-data center interconnections. This chapter discusses the analog and digital IM-DD optical communication systems with different high-order modulation formats. Typically, the direct modulation of Fabry–Perot lasers at lower cost don’t meet the stringent linearity requirements. In comparison with direct modulation alternatives, the drawbacks faced while using MZM-based systems are size and cost. At the receiver’s side, positive–intrinsic–negative (PIN)-type photodiode and avalanche photodiode (APDs) are widely used in direct-detection optical system to convert optical data into electrical form. There are many reports on PAM-8, as well as on PAM-16 signals in direct-detection system. Some advanced direct-detection technologies have been developed to compete with coherent optics. With the growing interests in KK receiver–based advanced direct-detection system, a lot of innovative works and interesting results have been published.