Quantum mechanics imposes a fundamental limit to the accuracy of physical measurements. This limit can now be reached in optical and microwave experiments. For interferometric length measurements, the project VIRGO (an Italo-French project) for the detection of gravitational waves has motivated a precise and thorough evaluation of ultimate quantum limits. Very precise measurements are also possible in microwave experiments but little work has been done about the effect of quantum fluctuations in such measurements. The aim of the thesis is to study theoretically the effect of quantum fluctuations in different kinds of optical and microwave measurements. Using the analogies between electrical and optical measurements, we can extend all the results which have already been obtained in optics to electrical systems. Electrical systems also presents some interesting specificities that we intend to exploit. Precise results are expected for quantum limits in electrical position measurement.