Subsoil strata in the most densely populated areas of the Netherlands typically contain peat layers. Consequently, knowledge is required about the complex mechanical behaviour of peats for the purposes of the geotechnical design and maintenance of infrastructure. A test site was set up for a dike reinforcement project, primarily in order to determine the operational undrained shear strength of peats. The field trials, and extensive laboratory and field probe testing, showed that, although margins of uncertainty in the individual measurements are large, the combination of different testing techniques provides a clear image of the strength behaviour of peats. Additional testing of large samples showed the relevance of size effects for peat tests. The back analysis of the tests produced operational undrained shear strength values that concur closely with laboratory and field probe testing. These test results provide a direction for future research targeting the development of constitutive models to predict the stress—strain behaviour of peats correctly.