Abstract Shield tunnel drives in both, overconsolidated clays soils and sedimentary rock with clay minerals, frequently suffer extremely from hindrance due to clogging. The problem of clogging can have varying degrees, from locally blocked disk cutter housings to completely clogged excavation chambers with a blocked cutterhead. The clogging risk depends on multiple mineralogical and soil-mechanical parameters. The authors have more than 15 years of experience in practice and scientific research with the problem of clay clogging of TBMs. The paper shows the occurrence and risk for clogging in varying ground types and for different machine components, summarizes the available characterization methods for the stickiness of the ground, discusses a variety of laboratory tests for the assessment of clogging potential, explains a recently developed diagram for the assessment of clogging risks for all types of tunnelling machines, and introduces a new testing scheme for the evaluation of sedimentary rocks regarding clogging.