This compilation paper revisits the Schengen set up including recentdevelopments triggered by both the migration crisis and terrorism, andsubsequently presents and qualifies expert estimates of the potential economicimpact stemming from the introduction of ID-checks, including a case study onthe very key transport sector. Estimates by The Kiel Institute for the WorldEconomy point at a burden on EU citizens and firms of between€7 and €14 billionfor the entire EU. TML-Leuven assessed the cost for transport users of reinstatingborder controls in the entire Schengen area to amount between€2.5 billion and€5 billion annually (waiting times 10-20 minutes for passenger cars and 30-60minutes for lorries). Effects, however, could prove much more costly if e.g. full-fledged border controls are permanently re-introduced. This paper has beenprepared by the Policy Department A at the request of the European Parliament's Committee on Internal Market and Consumer Protection (IMCO).