Apparatus operative to: analyse a sequence of video frames of a plurality of surveillance network camera view fields to define tracks taken by plural objects in first and second view fields; determine, in each of said first and second view fields, corresponding start and end points for each said track; determine start and end times for each said track in said first and second view fields based on said start and end points; store respective start and end points and said start and end times as attributes of each said track for said first and second view fields; determine a transition time model for transitions of objects between each respective pair of: exit zones in said first view field; and entrance zones in said second view field, wherein each said transition time model is based upon: a spatial distance between an exit and entry zone pair and a speed of an object; and a statistical distribution of transition times of observed tracks; store said transition time models; responsive to tagging of an object in said first view field, which object exits said first view field via a given exit zone, said tagging comprising selecting, on a display displaying an image representative of said first view field, a portion of said image representing said object, identify one or more tracks having start times in said second view field falling within a transition time window related to a model associated with said given exit zone and display said one or more tracks as candidate tracks; determine, for each of said one or more candidate tracks, an appearance of a candidate object associated with its respective candidate track as an attribute of said candidate object and compare the appearance attribute of each of the candidate objects with an appearance attribute of said tagged object to determine a probability value for each candidate object being the tagged object and to rank said one or more tracks in accordance with said probability value; characterised in that: said appearance of said object is based upon a colour model for said object in which the colour of an object is derived by splitting a bounding box for the object into four contiguous horizontal segments and determining the primary colour in each segment; and further characterised in that: said apparatus is operative to only display the most likely tracks. analyse a sequence of video frames of a plurality of surveillance network camera view fields to define tracks taken by plural objects in first and second view fields; determine, in each of said first and second view fields, corresponding start and end points for each said track; determine start and end times for each said track in said first and second view fields based on said start and end points; store respective start and end points and said start and end times as attributes of each said track for said first and second view fields; determine a transition time model for transitions of objects between each respective pair of: exit zones in said first view field; and entrance zones in said second view field, wherein each said transition time model is based upon: a spatial distance between an exit and entry zone pair and a speed of an object; and a statistical distribution of transition times of observed tracks; exit zones in said first view field; and entrance zones in said second view field, a spatial distance between an exit and entry zone pair and a speed of an object; and a statistical distribution of transition times of observed tracks; store said transition time models; responsive to tagging of an object in said first view field, which object exits said first view field via a given exit zone, said tagging comprising selecting, on a display displaying an image representative of said first view field, a portion of said image representing said object, identify one or more tracks having start times in said second view field falling within a transition time window related to a model associated with said given exit zone and display said one or more tracks as candidate tracks; determine, for each of said one or more candidate tracks, an appearance of a candidate object associated with its respective candidate track as an attribute of said candidate object and compare the appearance attribute of each of the candidate objects with an appearance attribute of said tagged object to determine a probability value for each candidate object being the tagged object and to rank said one or more tracks in accordance with said probability value; characterised in that: said appearance of said object is based upon a colour model for said object in which the colour of an object is derived by splitting a bounding box for the object into four contiguous horizontal segments and determining the primary colour in each segment; and further characterised in that: said apparatus is operative to only display the most likely tracks.