Multiple interactions at HERA

Presented are recent measurements in ep collisions at HERA, which comprise results that are expected to be sensitive to multiple interactions and the underlying event. A measurement of 3- and 4-jet event cross-sections in photoproduction, as well as two measurements which target the activity in the soft underlying event in photoproduction and deep inelastic scattering at low photon virtuality (Q2), respectively, are reviewed. The data are compared to various QCD models with and without multiple interactions simulated. The multijet event cross-sections are in addition compared to a fixed-order calculation.