We discuss concepts for high-throughput (effective area 250-1400 cm2), high-resolution (spectral resolving power R > 3500) soft X-ray grating spectroscopy in missions of moderate (probe-class or smaller) scale. Such missions can achieve high-priority scientific objectives identified by the Astro2010 Decadal Survey attainable in no other way, and would provide an essential complement to any future large-area X-ray observatory equipped with non-dispersive spectrometers. We enumerate key science drivers and discuss consequences of various alternative design choices for scientific capability and overall mission size.
