Considerable interest is being shown in the low-NO(x) combustion of natural gas, and this study concerns two approaches to the production of low-NO(x) combustion units. In the first part the combustion of partly premixed natural gas/air mixtures in two domestic water heaters was experimentally investigated. Measurements of gas temperature and stable species concentrations were taken in and around the combustion zone, and from the flue. The first appliance was a bladed burner, commonly used in domestic water heaters, of maximum thermal input of 11 kW in a combustion chamber 18 x 18 x 16 cm; the primary mixture fuel: air equivalence ratio was 1.92 inside the burner body. The second appliance was fan-assisted and included a flue-gas valve allowing 0-15% of the combustion products to be recirculated; its burner was of the horizontal bar type with two lines of primary mixture ports each firing toward plates forming a small angle with the burner body. Transverse slots in the plates allowed for secondary aeration, and the flames developed along the slots. From an understanding of the temperature/species profiles within the combustion zone, the main routes of NO formation could be determined. Thermal and prompt-NO formation routes were considered in both appliances, as combustion temperatures exceeded 1800 K and because of the fuel-rich nature of the primary flame zone. It can be assumed that an important role is played by the prompt-NO mechanism in the bar burner appliance, since the temperature did not reach 2000 K. In order to analyse the experimental results, the appliances were modelled by means of a commercial computational fluid dynamic package, together with a NO(x) post-processing package. In the second part, the use of natural-gas catalytic combustors as a future means of ultra-low NO(x) combustion was investigated. Particular detail was placed in the design of combustors and in the choice of catalyst, because of poisoning and high-temperature de-activation. Experimental measurements were coupled with theoretical modelling by means of the Sandia National Laboratories code PREMIX.