In a grid architecture, a grid workflow management system is a type of high-level grid middleware which is supposed to support modelling, redesign and execution of large-scale sophisticated scientific and business processes in many complex e-science and e-business applications. To ensure the correctness of grid workflow specification and execution, grid workflow verification and validation must be conducted. However, current research on grid workflow verification and validation is at the infancy stage and very few projects focus on them. Therefore, a systematic investigation and an overall classification of key issues in grid workflow verification and validation is helpful and should be presented so that we can keep on the right track and reduce unnecessary work as much as possible. As such, in this paper, we analyse the grid workflow verification and validation and present a taxonomy. Especially, we identify some important open points which are not discussed by the current research and hence need further investigation. The taxonomy is aimed at providing an overall picture of grid workflow verification and validation.