Pore-throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale. That continuum is documented in this article using previously published data on the pore and pore-throat sizes of conventional reservoir rocks, tight-gas sandstones, and shales. For measures of central tendency(mean,mode,median),pore-throatsizes(diameters) are generally greater than2 mm in conventionalreservoir rocks, range from about 2 to 0.03 mm in tight-gas sandstones, and rangefrom0.1to0.005 mminshales.Hydrocarbonmolecules, asphaltenes, ring structures, paraffins, and methane, form another continuum, ranging from 100 A ˚ (0.01 mm) for asphaltenes to 3.8 A ˚ (0.00038 mm) for methane. The pore-throat size continuum provides a useful perspective for considering (1) the emplacement of petroleum in consolidated siliciclastics and (2) fluid flow through fine-grained source rocks now being exploited as reservoirs.