  MAORY is the Multi-conjugate Adaptive Optics RelaY for the European ELT aimed
at providing a 1 arcmin corrected field to MICADO, a near-infrared
spectro-imager with a focus on astrometry. In this paper we re-view the main
requirements and analysis that justify the current adaptive optics architecture
and subsystem requirements. We discuss the wavefront error budget allocation
focusing on the worst offenders terms and on a statistical analysis of their
dependence on atmospheric and sodium profiles. We present an updated revision
of the trade-off studies on the main AO parameters that, along with
considerations coming from optical and mechanical subsystems, are used to
define the preliminary design of the instrument.
