Positron emission tomography (PET) is a technique for imaging the 3-dimensional distribution of radio-labelled molecules in vivo. This technique plays an increasingly important role in preclinical research involving mice and rats. The small dimensions of these animals impose stringent requirements on both the image resolution and the sensitivity of dedicated small-animal PET systems. Monolithic scintillator detectors enable the construction of PET systems with a system resolution comparable to that of existing systems, but with a strongly improved sensitivity. The detectors consist of a monolithic piece of scintillation material (e.g. LSO) read out by one or more position-sensitive solid state light sensors, such as avalanche photodiodes (APDs). The position of annihilation photons impinging on the detector is derived from the distribution of scintillation light on the light sensors. In this thesis, the performance characteristics of several prototype detectors are determined experimentally and analysed using empirical models. Detector spatial resolutions comparable to the current state-of-the-art are obtained. The spatial resolution is found to be independent of the angle of incidence of the annihilation photons, permitting the use of a thick layer of scintillation material to maximise sensitivity, without degrading the image resolution away from the centre of the field of view (FOV). This is confirmed by Monte-Carlo simulations of PET scanners based on these detectors. These simulations yield image resolutions which are uniform over the FOV, and sensitivities of more than 3 times the current state-of-the art.