This chapter is devoted to QKD fundamentals. The chapter starts with a description of the key differences between conventional cryptography, classical physical-layer security (PLS), and QKD. In the section on QKD basics, after a historical overview, we review different QKD types and briefly describe common postprocessing steps, namely information reconciliation and privacy amplification steps. In the same section, we provide two fundamental theorems on which QKD relies on, no-cloning theorem and the theorem of inability to unambiguously distinguish non-orthogonal quantum states. In the section on discrete variable (DV)-QKD systems, we describe in detail BB84 and B92 protocols as well as Ekert (E91) and EPR protocols. In the same section, the time-phase encoding protocol is also described. Regarding the BB84 protocols, different versions, suitable for different technologies, are described. In the section on QKD security, the secret-key rate (SKR) is represented as the product of raw key rate and fractional rate, followed by a description of different limitations to the raw key rate. Then, the generic expression for the fractional rate is provided, followed by a description of different eavesdropping strategies, including individual (independent or incoherent) attacks, collective attacks, coherent attacks, and the quantum hacking/side-channel attacks. For individual and coherent attacks, the corresponding secrete fraction expressions are described. The next section is devoted to various definitions of security, including the concept of ε-security. Then, the generic expressions for 2-D DV-QKD schemes are derived for both prepare-and-measure and decoy states-based protocols. To facilitate the description of continuous variable (CV)-QKD protocols, the fundamentals of quantum optics are introduced first. In the section on CV-QKD protocols, both squeezed states-based and coherent states-based protocols are described. Given that the coherent states are much easier to generate and manipulate, the coherent states-based protocols with both homodyne and heterodyne detections are described in detail. The secret fraction is derived for both direct and reverse reconciliation-based CV-QKD protocols. Furthermore, the details on practical aspects of GG02 protocol are provided. In the same section, the secret fraction calculation for collective attacks is discussed. Then, the basic concepts for measurement-device-independent (MDI)-QKD protocols are introduced. The final section provides some relevant concluding remarks.