The present disclosure provides a burner for increasing the temperature of a gas stream, such as an exhaust gas stream, to burn particulate matter and unburnt hydrocarbons within the exhaust gas. As well, by heating or increasing the temperature of the gas stream and combustion products exiting the burner, the gas stream is brought into a condition more suitable for additional treatment to further reduce exhaust gas emissions from the engine. The device includes an outer housing and a series of combustion chambers mounted within the outer housing for increasing and further developing combustion of the incoming gases within the series of combustion chambers. A primary application for the device is for use with diesel engines to burn particulate matter and unburnt hydrocarbons within the diesel emissions and/or act as a heat source for activating diesel particulate filters and/or other catalytic components or devices.