Microresonators are fundamental building blocks of any photonic integrated circuit, as they can be used as filters, modulators, sensors, and to enhance light emission. If these resonators are suspended and are free to oscillate, we can exploit the interaction of the optical and mechanical resonance. Demonstration of the fundamental interactions between the optical and mechanical fields, as well as applications based on this physics, were already presented for other material systems such as silicon, silicon oxide, or silicon nitride. Single crystal diamond is a strong candidate to realise high quality resonators due to the excellent material properties. The mechanical properties, such as stiffness, low intrinsic damping, and high thermal conductivity, are critical for mechanical oscillators with high frequency and high quality factor, which is strongly correlated to the noise of the oscillator. The wide transparency range and the low absorption enable operation in a large wavelength range spanning from near ultraviolet to far infrared. Diamond can host very bright emitters, based on defects of the diamond lattice, which can be employed as single photon sources, quantum memories, or very sensitive magnetic sensors. Chemical resistance to most acids or bases permit operations in aggressive environments. For these reasons, single crystal diamond is an attractive platform for integrated photonics and micro- or nanomechanics, and it should be possible to realize low noise optomechanical resonators. However, compared to more established material systems, microfabrication of single crystal diamond is not easy. High quality single crystal diamond substrates are available, to date, only as bulk plates, therefore it is important to develop fabrication strategies that isolate optically and mechanically a thin diamond layer from the rest of the diamond substrate. Over the last years, several approaches have been proposed and in this work two methods will be employed: 3D milling using a focused ion beam to create micrometer sized disks supported by a narrow pillar; and etching of the bulk diamond using a plasma which is selective to particular crystal planes of the single crystal diamond. Using these processes, single crystal diamond microresonators were realized, measuring optical quality factors of 5700 and 42000 at telecom wavelengths, respectively, and of 3100 and 10500 at visible wavelengths, respectively. Single crystal silicon is similar in many aspects to single crystal diamond, with the material properties being slightly worse. However, silicon benefits from decades of knowledge developed for integrated electronic circuits and integrated silicon photonics circuits. Silicon photonics is now extensively used in telecommunications to interface the optical links to electronics. Commercial foundries started to offer silicon photonics services, either within the CMOS fabrication or with dedicated processes. Micro Electro Mechanical Systems represent a way to expand on the capability of photonic integrated circuits by providing low power and/or reconfigurable components. In this context, optomechanical oscillators were realized by postprocessing an IMEC iSiPP50G silicon photonics chip. Optical quality factors of 300000 were measured at telecom wavelengths. Mechanical oscillations are supported at 250 MHz, with quality factors of 1800 measured at ambient conditions.