A method of determining content to be presented on a mobile device, the method comprising: receiving sensor data from a mobile device, wherein the sensor data comprises: first sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of the content of a first type at the mobile device over a first time period; and second sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of content of a second type at the mobile device over a second time period, wherein the content of a second type is different to the content of a first type and where the first and second time periods are consecutive; and determining the comparative level of user engagement with the content of the first type based on the first sensor data and the second sensor data; selecting additional content of the first type based on the determined comparative level of user engagement; and transmitting the selected additional content of the first type to the mobile device for output by the mobile device. receiving sensor data from a mobile device, wherein the sensor data comprises: first sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of the content of a first type at the mobile device over a first time period; and second sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of content of a second type at the mobile device over a second time period, wherein the content of a second type is different to the content of a first type and where the first and second time periods are consecutive; and determining the comparative level of user engagement with the content of the first type based on the first sensor data and the second sensor data; selecting additional content of the first type based on the determined comparative level of user engagement; and transmitting the selected additional content of the first type to the mobile device for output by the mobile device. first sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of the content of a first type at the mobile device over a first time period; and second sensor data relating to the positioning of the mobile device with respect to a line-of-sight between a user and a display device of the mobile device sensed by the mobile device during display of content of a second type at the mobile device over a second time period, wherein the content of a second type is different to the content of a first type and where the first and second time periods are consecutive; and determining the comparative level of user engagement with the content of the first type based on the first sensor data and the second sensor data; selecting additional content of the first type based on the determined comparative level of user engagement; and transmitting the selected additional content of the first type to the mobile device for output by the mobile device.