Because of the diversity in available questionnaires, it is not easy for researchers to decide which instrument is most suitable for his or her specific demands. Therefore, we systematically summarized and appraised studies examining measurement properties of self-administered and proxy-reported physical activity (PA) questionnaires in youth. Literature was identified through searching electronic databases (PubMed, EMBASE using 'EMBASE only' and SportDiscus) until May 2009. Studies were included if they reported on the measurement properties of self-administered and proxy-reported PA questionnaires in youth (mean age <18 years) and were published in the English language. Methodological quality and results of included studies was appraised using a standardized checklist (qualitative attributes and measurement properties of PA questionnaires [QAPAQ]). We included 54 manuscripts examining 61 versions of questionnaires. None of the included questionnaires showed both acceptable reliability and validity. Only seven questionnaires received a positive rating for reliability. Reported validity varied, with correlations between PA questionnaires and accelerometers ranging from very low to high (previous day PA recall: correlation coefficient [r] = 0.77). In general, PA questionnaires for adolescents correlated better with accelerometer scores than did those for children. From this systematic review, we conclude that no questionnaires were available with both acceptable reliability and validity. Considerably more high-quality research is required to examine the validity and reliability of promising PA questionnaires for youth.