Despite the increasing use of advanced imaging methods to pattern chip features, process windows continue to shrink with decreasing critical dimensions. Controlling the manufacturing process within these shrinking windows requires monitor structures designed to maximize both sensitivity and robustness. In particular, monitor structures must exhibit a large, measurable response to dose and focus changes over the entire range of the critical features process window. Any process variations present fundamental challenges to the effectiveness of OPC methods, since the shape compensation assumes a repeatable process. One particular process parameter which is under increasing scrutiny is focus blur, e.g. from finite laser bandwidth, which can cause such OPC instability, and thereby damage pattern fidelity. We introduce a new type of test target called the Process Monitor Grating (PMG) which is designed for extreme sensitivity to process variation. The PMG design principle is to use assist features to zero out higher diffraction orders. We show via simulation and experiment that such structures are indeed very sensitive to process variation. In addition, PMG targets have other desirable attributes such as mask manufacturability, robustness to pattern collapse, and compatibility with standard CD metrology methods such as scatterometry. PMG targets are applicable to the accurate determination of dose and focus deviations, and in combination with an isofocal grating target, allow the accurate determination of focus blur. The methods shown in this paper are broadly applicable to the characterization of process deviations using test wafers or to the control of product using kerf structures.