Harmonic tremor signals appear in seismological records in a far wider context than on volcanoes where they have first been described. Yet, the signals typically share common characteristics: Tremor usually shows an emergent onset and lasts considerably longer than impulsive earthquake signals. Durations range between minutes and months. Harmonic tremor spectra show distinct peaks with a fundamental frequency in many cases in the range of 0.5-5 Hz and a series of harmonic overtones. These frequencies may glide over time, giving seismological tremor records converted into the audible frequency range the appearance of melodic songs. Other commonly observed characteristics are an inverse relation between the tremor amplitude and the fundamental frequency, period doubling and sudden switching into a non-harmonic, chaotic mode. The underlying sources and source mechanisms, however, may be very different: Here, I will present harmonic tremor signals caused by Antarctic icebergs by periodic stick-slip quakes during collision or by fluid flow through narrow cracks. The latter mechanism is frequently used to explain harmonic tremor in volcanoes. I will further present the properties of tidally modulated tremor presumably caused by submarine hydrothermal circulation on a mid-ocean ridge. These tremor signals may easily be confused with harmonic tremor excited by bottom currents acting on the structure of ocean bottom seismometers and thus causing unwanted disturbances in many seafloor seismological records. Finally, even the vibrations of ship hulls couple efficiently into the ground and appear on ocean bottom seismometer records as prominent harmonic tremor.