Methods and apparatus are disclosed for selecting encoding specifications for encoding audio and/or video data to be streamed from a sender to a receiver via a network. Methods and apparatus for encoding data using a selected encoding specification, and for streaming data which has been encoded using a selected encoding specification from a sender to a receiver via a network are also disclosed. The selecting method comprises selecting an encoding specification (s930) in dependence on performance measures (86) previously obtained using a plurality of different encoding specifications in respect of a monitored network when in each of a plurality of different network conditions (87), respective network conditions being characterised by different combinations of data-transmission characteristics.