The rank of a bimatrix game is the matrix rank of the sum of the two payoff matrices. For a game of rank r, the set of its Nash equilibria is the intersection of a generically one-dimensional set of equilibria of parameterized games of rank r − 1 with a hyperplane. We comprehensively analyze games of rank one. They are economically more interesting than zero-sum games (which have rank zero), but are nearly as easy to solve. One equilibrium of a rank-1 game can be found in polynomial time. All equilibria of a rank-1 game can be found by path-following, which finds only one equilibrium of a bimatrix game. The number of equilibria of a rank-1 game may be exponential. We also present a new rank-preserving homeomorphism between bimatrix games and their equilibrium correspondence.