Abstract: An apparatus for heating a flowing fluid with microwave energy comprising a conduit having an inlet portion, an outlet portion, and a retention portion wherein at least the retention portion of the conduit is fabricated from a material which is substantially microwave transparent. A temperature probe is provided for measuring the temperature of the flowing fluid in the outlet portion. The cross-sectional area of the retention portion is substantially greater than the cross-sectional area of the outlet portion such that the flow rate through the retention portion is slower than the flow rate through the outlet portion from the retention portion to the temperature probe. Microwave energy is used to heat the flowing fluid in the retention portion, the microwave energy being generated by a source which is controlled in response to a signal from the temperature probe. The retention portion of the conduit resides within a wave guide and passes transversely therethrough.