Retention is a general term for the process of keeping fine particles and fiber fines within the web of paper as it is being formed.

The word also can be used as an adjective, as in "a high level of retention was achieved on this paper machine." Retention can occur by various mechanisms.

The simplest of these is mechanical sieving by the forming fabric. Once a fiber mat begins to form, the mat itself usually can act as a much more effective and finer sieve than the forming fabric.

But even then, sieving does not effectively retain particles less than about 10 micrometers in size. Rather, retention of fine particles requires the action of colloidal forces, including polymeric bridging or a charged patch mechanism.

Retention aid chemicals can be effective either by attaching fine particles to fiber fines or fibers or by agglomerating them so that they can be sieved more effectively.