Over 70 billion chickens are slaughtered globally each year. Almost all are meat breeds, typically housed in very large barns with a litter floor. Recently however, modern cage systems have been developed which provide very limited space and stack several tiers high. There is debate about the impacts of such modern cage systems on chicken welfare. Accordingly, we systematically reviewed studies assessing the welfare of meat chickens kept in either modern cage systems or littered barns. Overall, 23 studies were reviewed, and none of the experimental studies reviewed incorporated a full behavioural analysis. Therefore, significant concerns exist about the deprivation of natural behaviours in meat chickens kept in modern cage systems. Given the numbers of meat chickens globally that could be impacted by these modern cage systems, this is a major animal welfare concern.