The paper presents an overview of Quality of Service (QoS) definitions, and reviews approaches to identifying quality requirements for audio and video quality in the context of novel multimedia services. We advocate an approach that maximizes ecological validity. It is important to recruit participants from the expected user population, create realistic tasks and using real material, and use an assessment method that creates minimal load on study participants. Finally, it is important to replicate all physical factors that affect quality in the eventual context of use. We present an example of how we apply this approach to identifying video quality requirements for mobile TV viewing.