ERA-NET ROAD initiated a transnational research project titled "Optimization of thin asphalt layers". The DRI-BRRC-VTI Consortium was trusted with carrying out the project and began with a State-of-the-Art report covering, among other things, a literature study and an inventory of experience with using thin asphalt layers (TAL). The results of this phase of the project are given in the present report. This study was limited to thin asphalt mixtures with a maximum thickness of 30 mm, which means that surface dressings or slurry seals were outside the scope of the project. Neither were top layers of double-layer porous pavements considered as TAL, even though such top layers often are 20-30 mm thick. Mix design and optimization was the subject of another study in this project and is therefore not treated here.