  Crowding is most likely an important factor in the deterioration of strategy
performance, the increase of trading costs and the development of systemic
risk. We study the imprints of \emph{crowding} on both anonymous market data
and a large database of metaorders from institutional investors in the U.S.
equity market. We propose direct metrics of crowding that capture the presence
of investors contemporaneously trading the same stock in the same direction by
looking at fluctuations of the imbalances of trades executed on the market. We
identify significant signs of crowding in well known equity signals, such as
Fama-French factors and especially Momentum. We show that the rebalancing of a
Momentum portfolio can explain between 1-2\% of order flow, and that this
percentage has been significantly increasing in recent years.
