This chapter is devoted to covert communications, also known as low-probability of detection/intercept, as well as stealth communications, and how they can improve secret-key rate for QKD applications. The chapter starts with a brief introduction to covert communications, followed by description of their differences with respect to steganography. One of the key technologies to enable covert communication over wireless channels, the spread spectrum concept, is introduced next. After that, the rigorous treatment of covert communication over an additive white Gaussian noise channel is provided, and the square root law is derived. The importance of hypothesis testing in covert communications is discussed as well. The covert communication over the discrete memoryless channels is discussed after that. The next topic is related to different approaches to overcome the square root law, including the use of friendly jammer that varies the noise power so that the square root law can be overcome, and positive covert rate be achieved. The concept of effective secrecy is introduced next, a recent secrecy measure, whose definition includes both strong secrecy and stealth communication conditions. After that, the covert/stealth communication concept is applied to optical communication systems. We then describe how the covert concept can be applied to information reconciliation step in QKD to simultaneously improve secret-key rate and extend the transmission distance. Experimental low probability of intercept and covert communication results are provided next. Finally, the set of problems is provided for a reader to be able to get deeper understanding of the underlying concepts.