This thesis describes the intrinsic limits of superconducting microresonator detectors. In a superconductor at low temperature, most of the electrons are paired into so called Cooper pairs, which cause the well-known electrical conduction without resistance. Superconducting microwave resonators have therefore very high quality factors and are sensitive to the smallest signals. A small change in the number of Cooper pairs can therefore be detected with these resonators. The binding energy in a superconductor, such as aluminium, is low, which makes such a detector suitable for detection of submillimetre radiation, a wavelength range that is particularly interesting for astronomy. Each resonator can have a slightly different length, and thus a different resonant frequency, which enables the readout of thousands of resonators (pixels) with a single readout line. Because the detector principle relies on counting the number of broken Cooper pairs (so called quasiparticles), the most fundamental source of noise of these resonator detectors is due to fluctuations in the number of quasiparticles. We demonstrate that superconducting aluminium resonators are indeed limited by these fluctuations. We show that a measurement of these fluctuations also provides access to the number of quasiparticles and their lifetime, which are intrinsic properties of the superconductor itself. At decreasing temperatures the number of quasiparticles is expected to decrease exponentially. However, we observe that the number of quasiparticles and their lifetime saturate at low temperature. Measurements of these quasiparticle fluctuations as a function of the microwave readout power revealed that the excess quasiparticles at low temperature are due to microwave power dissipation. The second main topic of this thesis is therefore how the microwave readout signal affects the superconductor and the resonator response. We show that the resonator response becomes strongly nonlinear due to readout power absorption. Since the microwave photon energy is lower than the pair-breaking energy, the microwave signal should not be able to break Cooper pairs. We show that the creation of excess quasiparticles is due to an intricate effect, the redistribution of quasiparticles over energy due to readout power absorption. Finally, we report an experiment in which we apply terahertz radiation to a microwave resonator detector. To demonstrate that the detector is sensitive enough to be used in a camera aboard a cooled space telescope, the conditions for such an instrument have to be mimicked in the test-setup. We observe that the noise of the detector is limited by photon-noise, fundamental fluctuations due to the source of radiation, and that the sensitivity fulfils the requirements for a space telescope.