Waves at the electron plasma frequency are found throughout the heliosphere. They provide indicators of unstable electron distributions, are routinely used to estimate the local electron number density, and can lead to radio wave emission at the plasma frequency and its harmonics. Although they have been studied extensively in various solar and heliospheric plasma regions, there is a lack of statistical studies of plasma frequency waves in Earth's magnetotail. Here, the occurrence and properties of plasma frequency waves, namely Langmuir and upper hybrid (UH) waves, are investigated in Earth's magnetotail using the four Magnetospheric Multiscale spacecraft. In Earth's magnetotail plasma frequency waves are observed about 1% of the time. About 80% of the waves are identified as Langmuir waves, while about 20% are identified as UH waves. The waves are primarily found in the plasma sheet boundary layer. By comparing with the local electron distributions it is shown that the Langmuir waves are generated by the bump-on-tail instability, while UH waves are typically associated with broad electron beams or loss-cone-like distributions. The majority of the waves are found in close proximity to ion outflow regions associated with magnetic reconnection in the magnetotail. The waves are likely generated by plasma sheet electrons escaping along newly reconnected magnetic field lines or electron beams propagating toward the distant magnetotail.