(Joint work with L. Cotta-Ramusino and R. Manning) DNA interactions with proteins frequently involves looping in which the location and orientation of the two ends of a DNA segment are prescribed. I will show how path integral methods can be used to obtain a sequence-dependent formula for the probability of loop formation, including the case of minicircle cyclization. The expression involves the minimal energy path, a nonlinear computation, with a correction for fluctuations in terms of certain Jacobi fields, a linear computation.