Ultra-High-Q microresonators enable the confinement of extremely high levels of optical power into tiny micron-scale volumes. This allows to observe nonlinear optical effects at extremely low threshold powers, making microresonators ideal components for future generations of integrated photonic circuits and for optical computing. The aim of this EngD project is the investigation of nonlinear interaction between counter-propagating light in optical microresonators. First experimental results show symmetry breaking between clockwise and counter-clockwise circulating modes in microresonators. This symmetry breaking can be utilized to develop non-reciprocal optical devices like optical diodes and circulators. Moreover this effect can be used for rotation sensors with enhanced sensitivity and all-optical switches/transistors. The goal of the EngD project is to both theoretically and experimentally investigate this symmetry-breaking effect in micromachined resonators and develop proof of concept experiments for future applications.