  Adaptive Optics at mid-IR wavelengths has long been seen as either not
necessary or easy. The impact of atmospheric turbulence on the performance of
8-10 meter class telescopes in the mid-IR is relatively small compared to other
performance issues like sky background and telescope emission. Using a
relatively low order AO system, Strehl Ratios of larger than 95% have been
reported on 6-8 meter class telescopes. Going to 30-42 meter class telescopes
changes this picture dramatically. High Strehl Ratios require what is currently
considered a high-order AO system. Furthermore, even with a moderate AO system,
first order simulations show that the performance of such a system drops
significantly when not taking into account refractivity effects and atmospheric
composition variations. Reaching Strehl Ratios of over 90% at L, M and N band
will require special considerations and will impact the system design and
control scheme of AO systems for mid-IR on ELTs. In this paper we present an
overview of the effects that impact the performance of an AO system at mid-IR
wavelengths on an ELT and simulations on the performance and we will present a
first order system concept of such an AO system for METIS, the mid-IR
instrument for the E-ELT.
