Understanding of the chemical and physical processes that govern formation and destruction of nitrogen oxides (NOx) in combustion of solid fuels continues to be a challenge. Even though this area has been the subject of extensive research over the last three decades, there are still unresolved issues that may limit the potential of primary measures for NOx control. In most solid fuel fired systems oxidation of fuel-bound nitrogen constitutes the dominating source of nitrogen oxides. The present paper reviews some fundamental aspects of fuel nitrogen conversion in these systems, emphasizing mostly combustion of coal since most previous work deal with this fuel. However, also results on biomass combustion is discussed. Homogeneous and heterogeneous pathways in fuel NO formation and destruction are discussed and the effect of fuel characteristics, devolatilization conditions and combustion mode on the oxidation selectivity towards NO and N-2 is evaluated. Results indicate that even under idealized conditions, such as a laminar pulverized-fuel flame, the governing mechanisms for fuel nitrogen conversion are not completely understood. Light gases, tar, char and soot may all be important vehicles for fuel-N conversion, with their relative importance depending on fuel rank and reaction conditions. Oxygen availability and fuel-nitrogen level are major parameters determining the oxidation selectivity of fuel-N towards NO and N-2, but also the ability of char and soot to reduce NO is potentially important. The impact of fuel/oxidizer mixing pattern on NO formation appears to be less important in solid-fuel flames than in homogeneous flames. (C) 2003 Elsevier Science Ltd. All rights reserved.