A method performed by one or more data processing apparatus, the method comprising: processing a video comprising a plurality of video frames to generate, for each video frame of the plurality of video frames, a corresponding optical flow image characterizing a displacement of each pixel of the video frame between the video frame and a subsequent video frame in the video; for each optical flow image, processing the optical flow image using an optical flow object localization system (104; 120) to generate object localization data defining locations of objects depicted in the video frame corresponding to the optical flow image; and using: (i) the plurality of video frames, and (ii) the object localization data generated by the optical flow object localization system by processing the optical flow images corresponding to the plurality of video frames, training a novel object localization system to process a video frame to generate object localization data defining locations of novel objects depicted in the video frame; wherein the optical flow object localization system is trained using: (i) optical flow images, and (ii) object localization data defining locations of objects depicted in video frames corresponding to optical flow images; wherein the object localization data defining locations of objects depicted in a video frame corresponding to an optical flow image is obtained by processing the video frame using a known object localization system, wherein the known object localization system is configured to process the video frame to generate data defining locations of objects depicted in the video frame that are from a predetermined set of object classes; and wherein a novel object is an object for which the location of the object is included in the object localization data generated by the optical flow object localization system (104; 120), and is not included in the object localization data generated by the known object localization system. processing a video comprising a plurality of video frames to generate, for each video frame of the plurality of video frames, a corresponding optical flow image characterizing a displacement of each pixel of the video frame between the video frame and a subsequent video frame in the video; for each optical flow image, processing the optical flow image using an optical flow object localization system (104; 120) to generate object localization data defining locations of objects depicted in the video frame corresponding to the optical flow image; and using: (i) the plurality of video frames, and (ii) the object localization data generated by the optical flow object localization system by processing the optical flow images corresponding to the plurality of video frames, training a novel object localization system to process a video frame to generate object localization data defining locations of novel objects depicted in the video frame; wherein the optical flow object localization system is trained using: (i) optical flow images, and (ii) object localization data defining locations of objects depicted in video frames corresponding to optical flow images; wherein the object localization data defining locations of objects depicted in a video frame corresponding to an optical flow image is obtained by processing the video frame using a known object localization system, wherein the known object localization system is configured to process the video frame to generate data defining locations of objects depicted in the video frame that are from a predetermined set of object classes; and wherein a novel object is an object for which the location of the object is included in the object localization data generated by the optical flow object localization system (104; 120), and is not included in the object localization data generated by the known object localization system.