In contrast
with operational high brightness electron accelerators, axi-symmetric
systems operating normalized beam emittances at one mm-mrad level, all
presently operating Energy Recovery Linacs (ERLs) produce an order of
magnitude larger emittances in 10-to-20 mm mrad range. One of the reasons
is the use of dipole magnets, which mix transverse and longitudinal
degrees of freedom, and, consequently, can provide for conditions violating
emittance compensation conditions. In this paper we survey the reasons
of emittance growth in the operational ERLs. We discuss the theoretical
aspects of emittance preservation in ERLs and present some examples
of such schemes designed for BNL's R&D ERL. We compare performances
of the BNL's ERL system with the most popular ERL designs.