Abstract. The genus-1 KP-Whitham system is derived for both variants of the KadomtsevPetviashvili (KP) equation; namely the KPI and KPII equations. The basic properties of the KPWhitham system, including symmetries, exact reductions, and its possible complete integrability, together with the appropriate generalization of the one-dimensional Riemann problem for the Korteweg-deVries equation are discussed. Finally, the KP-Whitham system is used to study the linear stability properties of the genus-1 solutions of the KPI and KPII equations; it is shown that all genus-1 solutions of KPI are linearly unstable while all genus-1 solutions of KPII are linearly stable within the context of Whitham theory.