This paper is the product of one of the three working groups at Dagstuhl event 11382. The group was charged with producing a reference model for the process of longitudinal data production and use, with an emphasis on the specification and management of the supporting metadata. This model is designed to be useful for the gamut of study types where data are collected across time, including panel studies and repeated cross-sectional studies. It should also be useful for single cross-section studies.