Pores refer to the void spaces in a rock; porosity is a measurement that refers to the percentage of void spaces in a rock (the ratio of the volume of void spaces to the total volume of the rock sample). From an engineering perspective, porosity commonly is subdivided into “total porosity” (all pores, regardless of whether they are interconnected or isolated) versus “effective porosity” (porosity that is interconnected plus, depending on usage, pores associated with clay-bound water). For hydrocarbon explorationists, porosity (especially effective porosity) and permeability are probably the most important properties of any rock. Pores, therefore, are discussed throughout this book, and this relatively short chapter will serve mainly to give examples of the range of pore types in the context of their classification. It should be noted that accurate petrographic identification of the amount of porosity and the types of pores in sedimentary rocks requires careful thin-section preparation (Pittman, 1992a) and is aided by the injection of colored and/or fluorescent dyes into pores prior to thin sectioning (e.g., Ruzyla and Jezek, 1987).