The Scrape-Off Layer (SOL) of tokamak plasmas is home to very strong plasma flows, which are still relatively poorly understood. Recent Mach probe measurements, at the top of the poloidal cross-section, have confirmed their existence in JET. The radial profile of the parallel flow Mach number, M||, rises from M|| ~ 0.2 in the separatrix vicinity to a peak of M|| ~ 0.5 in the near SOL for forward field operation (i.e. with the ion B¥B drift directed downwards and where positive flow corresponds to the direction from outer to inner divertor targets). In reversed field, the flow is observed to stagnate throughout the SOL except near the separatrix, where the flow increases again to the forward field value.