Apparatus for monitoring activation in a heart comprises a probe 101, a plurality of electrodes 102 supported on the probe and extending over a detection area of the probe, the detection area being arranged to contact a detection region of the heart. Each of the electrodes 102 is arranged to detect electrical potential at a respective position in the heart during movement of a series of activation wave fronts across the detection region. A processor is arranged to analyse the detected electrical potentials to identify a propagation direction of at least one of the wave fronts, and to generate an output indicative of that direction.