An anti-skid brake conrol system includes a sensor which produces a signal proportional to the acceleration of a braked wheel and a comparator that responds to the acceleration signal by producing a signal which undergoes a step change at set acceleration levels so as to divide successive deceleration/acceleration cycles into successive acceleration periods separated by intermediate periods. The output of the comparator is integrated to produce an output signal which changes in a progressive manner in one sense during each acceleration period and in the opposite sense during each intermediate period so as to vary in accordance with surface friction conditions. The output signal is used to vary a control parameter or function within the system in accordance with the surface friction conditions. The integrator may comprise a capacitor which is discharged so that its potential falls during each acceleration period from an initial predetermined level and is then charged during each intermediate period so that its potential rises, the charging rate being increased at a predetermined threshold potential below which low friction conditions are signalled.