Apart from the previously mentioned attacks that exploit the algorithmic weaknesses of logic locking techniques, logic locking is also vulnerable to the emerging class of side-channel attacks, which are the focus of this chapter. The chapter introduces four attacks on logic locking that exploit various side-channels to extract secret key. The differential power analysis attack utilizes the power consumption of a chip to determine the secret logic locking key. The test-data mining attack and the hill climbing attack determine the secret key from the test data. The de-synthesis attack extracts the key by leveraging the traces left in a netlist during logic synthesis.