A method of monitoring a human or animal subject comprising the steps of: capturing a video image of the subject consisting of a time series of image frames each image frame comprising a pixel array of image intensity values; analysing the video image to determine movement values quantifying movement of the subject based on temporal variations in the image intensity values; and outputting an indication of the movement of the subject based on the movement values; wherein the step of analysing the video image to determine movement values quantifying movement of the subject based on temporal variations in the image intensity values comprises: measuring the variation with time of the image intensity values at each of a plurality of positions in the video image to form a respective plurality of movement signals; detecting global temporal intensity variations from the temporal variations in the image intensity values in predefined areas of the video image; quantifying the temporal variability of each of the movement signals; determining a movement value for each of the movement signals by de-correlating the global temporal intensity variations from the quantified temporal variability of the respective movement signal; grouping the movement signals into a plurality of groups according to their position in the video image, and forming for each group a single representative movement value on the basis of the movement values of movement signals in the group; and wherein the step of outputting an indication of the movement of the subject comprises: determining movement as being present at the position in the video image of the group if the representative movement value of the group is above a predetermined threshold. capturing a video image of the subject consisting of a time series of image frames each image frame comprising a pixel array of image intensity values; analysing the video image to determine movement values quantifying movement of the subject based on temporal variations in the image intensity values; and outputting an indication of the movement of the subject based on the movement values; wherein the step of analysing the video image to determine movement values quantifying movement of the subject based on temporal variations in the image intensity values comprises: measuring the variation with time of the image intensity values at each of a plurality of positions in the video image to form a respective plurality of movement signals; detecting global temporal intensity variations from the temporal variations in the image intensity values in predefined areas of the video image; quantifying the temporal variability of each of the movement signals; determining a movement value for each of the movement signals by de-correlating the global temporal intensity variations from the quantified temporal variability of the respective movement signal; grouping the movement signals into a plurality of groups according to their position in the video image, and forming for each group a single representative movement value on the basis of the movement values of movement signals in the group; and measuring the variation with time of the image intensity values at each of a plurality of positions in the video image to form a respective plurality of movement signals; detecting global temporal intensity variations from the temporal variations in the image intensity values in predefined areas of the video image; quantifying the temporal variability of each of the movement signals; determining a movement value for each of the movement signals by de-correlating the global temporal intensity variations from the quantified temporal variability of the respective movement signal; grouping the movement signals into a plurality of groups according to their position in the video image, and forming for each group a single representative movement value on the basis of the movement values of movement signals in the group; wherein the step of outputting an indication of the movement of the subject comprises: determining movement as being present at the position in the video image of the group if the representative movement value of the group is above a predetermined threshold.