In this thesis, I report on the exploration of non-linear optical phenomena in silica whispering gallery mode (WGM) microcavities. In particular, optomechanical interactions, and the generation of optical frequency combs due to the Kerr non-linearity of silica are investigated. Radiation pressure couples the optical field stored in a microcavity to mechanical degrees of freedom of its boundary. Our systematic survey of the mechanical modes present in silica WGM microcavities has enabled engineering devices which exhibit strong optomechanical coupling between highquality, radio-frequency (30–120 MHz) mechanical radial-breathing modes and ultra-high finesse optical WGMs. The finite build-up time of the intracavity field leads to complex dynamics in the optomechanical interaction. We observe and analyze the effect of dynamical backaction, in which the radiation-pressure force modifies the dynamics of the mechanical mode. In particular, we demonstrate for the first time, how this effect can be exploited to optically cool a mechanical mode. Quantum-noise limited optical interferometry is employed for the measurement of mechanical displacement fluctuations, from which the effective temperature of the mode is inferred. An imprecision at the level of 1 · 10 m/ √ Hz is reached, below the expected quantum mechanical zeropoint position fluctuations of the mechanical mode. For the first time, we demonstrate laser cooling also in the “resolvedsideband regime”, in which the optical photon storage time exceeds the mechanical oscillation period, as required for ground-state cooling. Technical sources of heating are eliminated by using low-noise lasers, a He-cryogenic environment for the experiment and suppressing laser absorption. Occupations down to 〈n〉 = 63 ± 20 mechanical excitation quanta are achieved in this manner. Simultaneously, we are able to assess the perturbation of the mechanical mode due to the process of measurement (measurement backaction). The optical techniques employed here are shown to operate in a near-ideal manner according to the principles of Quantum Measurement, displaying a backaction-imprecision product close to the quantum limit. In an independent set of experiments, we show that the high intensities circulating in silica WGM microcavities give rise to strong optical four-wave mixing due to the material’s Kerr nonlinearity. Starting from a continuouswave pump laser, broad, discrete optical spectra are formed by a cascade of optical sidebands to the pump. These “comb” spectra span more than 500 nm, and consist of lines spaced roughly by the cavity’s free spectral range. An optical frequency comb based on a mode-locked laser is used as a reference to determine the homogeneity of the microcavity comb lines’ spacing in the frequency domain. The microcavity comb lines are found to be equidistant at a relative level of 7 · 10, in spite of the presence of cavity dispersion. We have therefore demonstrated a novel, ultracompact source of optical frequency combs with applications in astronomy and telecommunication technology.