Quantum Cryptography allows for secure communication with the reliability of a natural law. It resembles the most advanced part of Quantum Information when practical means are concerned. The technology for Quantum Cryptography is currently taking the step from laboratory to industrial products; there are already companies producing in this sector. In this talk we address mainly the fundamentals of Quantum Cryptography with single photons, which are the reference standard for practical applications. Other techniques are also considered, like weak coherent pulses, continuous variables and entanglement based Cryptography schemes.