A method of collecting behavioural data of a computer user at a client computer during playback of media at the client computer, the client computer being in communication over a network with a remote ad server, the method comprising: receiving, at the remote ad server from a video player application running on the client computer, a call for a video ad response; sending, from the remote ad server to the client computer, the video ad response, wherein the video ad response is compliant with a Video Ad Standard Template specification; executing the video ad response in a runtime environment of the video player application running on the client computer, characterised in that: the video ad response includes a resource identifier that calls an emotion tracking application from a remote resource store, whereby the emotion tracking application is transferred and executed within the runtime environment of the video player application, and the method further includes: upon receipt by the video player application of an indication that the emotion tracking application is ready, playing back, by the video player application or the emotion tracking application, media on the client computer; and collecting, by the emotion tracking application, behavioural data that comprises information indicative of the computer user's emotional state during the playback of the media. receiving, at the remote ad server from a video player application running on the client computer, a call for a video ad response; sending, from the remote ad server to the client computer, the video ad response, wherein the video ad response is compliant with a Video Ad Standard Template specification; executing the video ad response in a runtime environment of the video player application running on the client computer, characterised in that: the video ad response includes a resource identifier that calls an emotion tracking application from a remote resource store, whereby the emotion tracking application is transferred and executed within the runtime environment of the video player application, and the method further includes: upon receipt by the video player application of an indication that the emotion tracking application is ready, playing back, by the video player application or the emotion tracking application, media on the client computer; and collecting, by the emotion tracking application, behavioural data that comprises information indicative of the computer user's emotional state during the playback of the media. the video ad response includes a resource identifier that calls an emotion tracking application from a remote resource store, whereby the emotion tracking application is transferred and executed within the runtime environment of the video player application, and the method further includes: upon receipt by the video player application of an indication that the emotion tracking application is ready, playing back, by the video player application or the emotion tracking application, media on the client computer; and collecting, by the emotion tracking application, behavioural data that comprises information indicative of the computer user's emotional state during the playback of the media. upon receipt by the video player application of an indication that the emotion tracking application is ready, playing back, by the video player application or the emotion tracking application, media on the client computer; and collecting, by the emotion tracking application, behavioural data that comprises information indicative of the computer user's emotional state during the playback of the media.