Condition monitoring enables the system manager to have an improved understanding of a machine’s condition under changing operational conditions. It can minimize the time and costs that are spent in maintenance activities, especially in applications where system performance and reliability are important. The objective of this thesis is to improve the knowledge of forced gear mesh vibrations of planetary gearboxes and therefore contribute to enhancing condition monitoring. This is accomplished by proposing two hypotheses. First, all publications on the subject assume that the effect of the time varying signal propagation delay between the main vibration source - the gear mesh point - and the (usually fixed) transducer can be ignored for condition monitoring purposes. Therefore the effect of the propagation delay is modelled as a phase modulation of the gear mesh vibration. The results can prove for the first time that the amplitude of the sidebands from the phase modulation is negligible and would not interfere with condition assessment based on analysis of the modulation of the gear mesh frequency, and thus the propagation delay can be neglected for practical purposes. A second hypothesis is also formulated and investigated in this work: internal vibration measurements can detect gear mesh faults that cannot be detected with external measurements. This is done by placing the vibration transducer on the rotating parts inside the planetary gearbox. The results of the internal measurement set-up shows an improved sensitivity for fault detection of planetary gearboxes by allowing the transducer to rotate on the planet carrier.