The Double Pulsar (PSR J0737-3039) consists of two active radio pulsars (labeled as A and B) that are bound in a slightly eccentric, tight orbit. This is one of the best astrophysical laboratories to test Einstein’s theory of general relativity. All five post-Keplerian parameters are measured independently for the A and the B pulsars, and they are consistent with predictions from general relativity. The Double Pulsar is also useful for understanding the formation and the coalescence of double-neutron-star systems. We summarize the properties of the Double Pulsar and discuss their implications for testing general relativity and for detecting gravitational waves.