<font size="2" face="Arial, Helvetica, sans-serif">Nonlinear optical frequency conversion processes in dispersive media are usually treated by assuming energy conservation between the interacting photons in the presence of a dispersion induced momentum mismatch. This momentum mismatch greatly restricts the efficiency of nonlinear frequency conversion processes. Quasi phase matching (QPM) compensates for this phase mismatch by spatially modulating the medium so that momentum is conserved, to within discrete values related to the geometry of the modulation. QPM enables not only efficient frequency conversion, but also many diverse applications such as beam and pulse shaping, multi-harmonic generation, high harmonic generation, all-optical processing and the generation of entangled photons. This makes QPM an important tool of photonics research and applications. Since its introduction in the early 1960s at the birth of nonlinear optics, QPM has always been implemented in the form of spatial modulation that is used to mitigate the momentum mismatch. Here we present a fundamental generalization of QPM where spatiotemporal nonlinear optical diffraction allows for both momentum and energy mismatch to be corrected-for. We note that a recent experiment proves the feasibility of spatiotemporal QPM</font>