Abstract. We introduce a refinement of the GPY sieve method for studying prime k-tuples and small gaps between primes. This refinement avoids previous limitations of the method, and allows us to show that for each k, the prime k-tuples conjecture holds for a positive proportion of admissible k-tuples. In particular, lim inf n (p n+m − p n ) < ∞ for any integer m. We also show that lim inf(p n+1 − p n ) ≤ 600, and, if we assume the Elliott-Halberstam conjecture, that lim inf n (p n+1 − p n ) ≤ 12 and lim inf n (p n+2 − p n ) ≤ 600.