A divisor in the moduli space \overline{M}_{g,n} of stable, n-pointed curves of genus g is conjecturally ample if and only if it positvely intersects a class of smooth, rational curves called F-curves. I will describe the F curves and explain why they are thought to specify all effective curves on \overline{M}_{g,n}. I will also discuss the surprising fact that if the conjecture is true for \M_{0,g+n} then it is true for \overline{M}_{g,n}.