The twin paradox of special relativity formulated in the geometrical setting of general relativity gives rise to the problem of determining the longest timelike curve between a given pair of points. As a first step one solves the local problem for a bundle of nearby curves (geodesics) in terms of Jacobi fields and conjugate points. These provide important information about geometrical properties of the given spacetime. The second step, to determine the globally maximal length curve in the set of all timelike curves with common endpoints, is harder and may be effectively performed only in spacetimes with high symmetries.