ST40 is a high-field, low-aspect-ratio spherical tokamak with a major radius of 0.4 m, aspect ratio of 1.8 and toroidal field of 2.3 T. A unique aspect of ST40 is the routine use of merging compression (MC) startup, in which the breakdown is achieved by forming two plasma rings around toroidal in-vessel coils. The resulting very high loop voltages (> 50 V) can produce a population of suprathermal electrons as a seed for runaways. Additionally, the hot-ion-mode scenarios explored in ST40 feature low densities, that can further increase the runaway electron population. Understanding and controlling the runaway electron generation has been critical for reaching the highest performance operating scenarios. In this contribution we present runaway electron observations in recent ST40 experiments, and first steps in modelling them, including DREAM simulations for runaway generation, ASCOT simulations for runaway confinement and SOFT synthetic diagnostics for IR synchrotron images.