  Based on the firm laws of physics rather than unproven foundations of
mathematical complexity, quantum cryptography provides a radically different
solution for encryption and promises unconditional security. Quantum
cryptography systems are typically built between two nodes connected to each
other through fiber optic. This chapter focuses on quantum cryptography systems
operating over free-space optical channels as a cost-effective and license-free
alternative to fiber optic counterparts. It provides an overview of the
different parts of an experimental free-space quantum communication link
developed in the Spanish National Research Council (Madrid, Spain).
