For the future lepton collider FCC-ee, different detector designs are studied and optimized. They must first achieve the required performances for heavy-flavour tagging, particle identification, tracking and particle-flow reconstruction, as well as lepton, jet, missing energy and angular resolutions, needed to successfully develop the very broad FCC-ee physics program, and exploit the extremely high statistical precision offered by this future collider. On top of that, they must all satisfy the constraints imposed by the challenging interaction region layout. FCC feasibility studies are being now carried out, using benchmark physics processes to determine, via appropriate simulations, the requirements on the detector performance which can guarantee that the systematic uncertainties of the measurements will be lowered as far as possible with the tiny statistical uncertainties as the target. Additionally, the potential for discovering very weakly coupled new particles, in decays of Z or Higgs bosons, motivates dedicated detector designs that would increase the efficiency for reconstructing the unusual signatures of such processes. These studies are crucial input to the further optimization of the two concepts described in the FCC-ee Conceptual Design Report, CLD and IDEA, and to the development of new concepts which might prove to be even better suited for the FCC-ee physics program.
