A monitoring system for the wet end of a paper or board machine includes at least one light fixture for illuminating pulp supplied onto a wire, at least one image sensor to be placed above the head box at the wet end of the machine, for capturing the activity of the pulp supplied from the head box in the wire section, in the direction of the movement of the wire; and a computing device, to which at least one image sensor is configured to transmit image information for determining activity data on the pulp, the computing device being configured to adjust the settings of the paper or board machine, and/or of the monitoring system, on the basis of the determined activity data. A method for monitoring the wet end of a paper or board machine by means of a monitoring system includes adjusting the settings of the paper or board machine and/or of the monitoring system, on the basis of the determined activity data.