Design for security not only involves the design of security functions which meet price, performance, power, and other constraints, but also resistance to attacks. An attack may be successful in extracting the secret key solely from the side channel signals recorded from a device executing a cryptographic algorithm. In many cases the plaintext of an encryption algorithm additionally may be required by the attacker (known plaintext attack) or may have to be controlled by the attacker (chosen plaintext attack). In some cases the knowledge of the ciphertext (ciphertext attack) along with the side channel signals may be sufficient for an attack. This chapter will introduce the side channel attack and the following chapter will discuss resistance techniques.