Dedicated to Hans Schneider on the occasion of his seventieth birthday.Abstract. Let K; h; i be a Hilbert space equipped with an inde nite inner product ; . Then K; ; is a complex Krein space. One can de ne the Krein space numerical range of an operator A acting on K as the collection of complex numbers of the form Av; v with v 2 K satisfying v;v = 1 . In this paper, the shapes of Krein space numerical ranges of operators in the complex plane using the joint n umerical range of self-adjoint operators on K; h; i are studied. Krein space numerical ranges of operators acting on a two-dimensional space are fully described. A Matlab program is developed to generate the sets in the nite dimensional case.