Today's time and frequency standards which are in active use ranging from the definition of the second to spacecraft applications are all based on atomic resonances in the microwave region controlling high-performance quartz crystal oscillators. The present status of these standards will be presented focusing in particular on cesium, hydrogen, and rubidium devices, as well as on new quartz crystal standards. A coherent picture of their physical principles and limitations is given based on the common attempt to reduce perturbations and Doppler effects. The sharpness of the resonance line is linked to the flicker noise stability limit of the various standards, and some speculation is made on where and how much further improvements may occur. It is demonstrated how new design concepts and new physical methods most likely will significantly improve the accuracy, stability, and practical usefulness of time and frequency standards.