Compared to conventional diesel fuel, use of biodiesel is generally found to reduce emissions of hydrocarbons (HC), carbon monoxide (CO), and particulate matter (PM); but to increase oxides of nitrogen (NOx) emissions. This paper reviews and summarizes relevant literature regarding the so-called "biodiesel NOx effect, and presents theories" to explain this effect. In modern diesel engines, several factors related to fuel composition and engine control strategies are important, though no single theory provides an adequate explanation of the biodiesel NOx effect under all conditions. There is evidence to suggest that effects on injection timing, ignition delay, adiabatic flame temperature, radiative heat loss, and other combustion phenomena all play some role. The biodiesel NOx effect can be mitigated by modifying engine control settings - particularly by retarding injection timing and increasing exhaust gas recirculation (EGR). The absolute magnitude of the biodiesel NOx effect appears to be reduced with modern engines, although there are cases where the percentage change is still substantial. Sophisticated after-treatment systems required to achieve the 2010 diesel engine emissions standards do not appear to be significantly affected by use of biodiesel. However, longer term study is warranted, as such systems have only been in commercial use for a short time. (C) 2012 Elsevier B.V. All rights reserved.