A classification of mudrocks for engineering purposes was considered necessary in order to improve the common qualitative use of a variety of geological terms. There has been a lack of agreement between authors on the precise definition of these terms. Mudrock is defined as a fine- to very finegrained, siliciclastic sediment or sedimentary rock. Quantitative boundaries for this group of rocks are proposed using meaningful soil and rock index parameters, at changes in mechanical or material behaviour. The basic division is between fine-grained engineering soil, non-durable rock and durable rock, at compressive strengths of 0.6 and 3.6 MN/m2. The mudrock soils are classified according to the British Standard Classification of Soils (BSCS) with the addition of the term silty clay. Fissile or anisotropic rocks are termed shales, including clayshales where non-durable. Isotropic ones are classed as siltstones, mudstones and claystones according to their quartz contents. Tectonically induced strength and fabric anisotropy can be important and may result in slaty shales where the rock microfabric has been well aligned. An upper compressive strength limit of 100 MN/m2 is proposed to distinguish diagenetic mudrocks from their metamorphic equivalents.