The International Linear Collider project develops a linear electron-positron collider with a first "Higgs factory" stage at 250 GeV, followed by an upgrade to higher energy. The precision physics program of the ILC places demanding requirements on the detectors that are to equip the interaction region. Extensive Monte Carlo simulations of complete detector concepts have been used to draw up the main specifications for the detector performance. A global design and R&D effort has addressed these challenging goals, with important progress in ultra-transparent vertex detector and tracker solutions and highly granular calorimeter systems. An overview will be given of the detector requirements and highlights of the R&D effort will be presented in this contribution.