This chapter is devoted to the discrete variable (DV) QKD protocols and represents the continuation of Chap. 7 . The chapter starts with the description of BB84 and decoy state-based protocols, and evaluation of their secrecy fraction performance in terms of achievable distance. The next topic is related to the security of DV QKD protocols when the resources are finite. We introduce the concept of composable ε-security and describe how it can be evaluated for both collective and coherent attacks. We also discuss how the concept of correctness and secrecy can be combined to come up with tight security bounds. After that we evaluate the BB84 and decoy state protocols for finite key assumption over atmospheric turbulence effects. We also describe how to deal with time-varying free-space optical channel conditions. The focus is then moved to high-dimensional (HD) QKD protocols, starting with the description of mutually unbiased bases (MUBs) selection, followed by the introduction of the generalized Bell states. We then describe how to evaluate the security of HD QKD protocols for finite resources. We describe various HD QKD protocols, including time-phase encoding, time-energy encoding, OAM-based HD QKD, fiber Bragg grating (FBGs)-based HD QKD, and waveguide Bragg gratings (WBGs)-based HD QKD protocols.