Despite the significant progress made in both theoretical and experimental quantum key distribution (QKD), the security of current QKD implementations is still not yet established rigorously. One of the major causes of this impasse is that existing security analyses are strongly dependently on the classical-post-processing block size, and as a consequence, the resulting security bounds are unduly pessimistic. In other words, applying these security bounds to current QKD implementations would severely impair the efficiency of these implementations. In this thesis, we overcome this impasse by providing a novel security framework that allows one to derive tight non-asymtptotic security bounds on a wide range of QKD implementations. This essentially closes the gap between theory and practice of QKD.