A theory of the Jovian decameter radiation is presented based on the assumed existence of beams of energetic electrons in the inner Jovian magnetosphere. Beam-like electron distributions are shown to be unstable to the growth of both upper hybrid and lower hybrid electrostatic waves. The upconversion of these waves to fast extraordinary mode electromagnetic radiation is calculated by using a fluid model. Two possibilities are considered. First, we make a random phase approximation which leads to a very conservative estimate of intensity that can be expected in decameter radiation. The alternative possibility is also considered, viz. that the upconversion process is coherent. A comparison of both processes suggests that an incoherent interaction may be adequate to account for the observed intensity of decametric radiation except perhaps near the peak of the spectrum (8 MHz). The coherent process is intrinsically more efficient and can easily produce the observed intensity near 8 MHz if only 0.01% of the energy in the beam is converted to electrostatic energy.