Biodiesel has received, and continues to receive, considerable attention for its potential use as an augmenting fuel to petroleum diesel. Its advantages include decreased net carbon dioxide, hydrocarbon, carbon monoxide, and particulate matter emissions, and fuel properties similar to petroleum diesel for ease of use in diesel engines. Its disadvantages include poorer cold flow characteristics, lower heating values, and mostly reported higher emissions of oxides of nitrogen (NO(x) = NO + NO(2), where NO is nitric oxide and NO(2) is nitrogen dioxide). This latter disadvantage (i.e., higher emissions of oxides of nitrogen) is the focus of this review article. NO(x) formation mechanisms are complex and affected by several different features (e.g., size, operating points, combustion chamber design, fuel system design, and air system design) of internal combustion engines. The slight differences in properties between biodiesel and petroleum diesel fuels are enough to create several changes to system and combustion behaviors of diesel engines. Combined, these effects lead to several complex and interacting mechanisms that make it difficult to fundamentally identify how biodiesel affects NO(x) emissions. Instead, it is perhaps better to say that several parameters seem to most strongly influence observed differences in NO(x) emissions with biodiesel, thus introducing several possibilities for inconsistency in the trends. These parameters are injection timing, adiabatic flame temperature, radiation heat transfer, and ignition delay. This article provides a review of the rich literature describing these parameters, and provides additional insight into the system responses that are manifested by the use of biodiesel. (C) 2010 Elsevier Ltd. All rights reserved.