The accelerating and frequently fluctuating price of conventional diesel, together with growing environmental
concerns has sparked renewed attention on the search for an alternative fuel. The awareness
of the toxic effects related to the tailpipe emissions of vehicles has driven many countries to look for a
less-polluted transportation fuel. In this regard, biodiesel (alkyl esters) from vegetable oils or animal fats
via transesterification is regarded as the most viable alternative as a green fuel for diesel engines. Transesterification
is a catalyzed process and, traditionally, homogeneous catalysts are employed. However,
this type of catalyst is not able to be reused and requires tedious washing and separating steps, hence,
stimulating the conception of heterogeneous-catalyzed transesterification. Despite the success of various
heterogeneous catalysts, many are not viable for wide industrial usage as most of the catalysts are expensive
and need additional preparation effort. Among them, CaO seems to have a promising place and the
increasing research on CaO is self-evidence of its capability in catalyzing the reaction. Therefore, in this
paper, various issues regarding CaO-catalyzed transesterification are reviewed. The diverse performance
of CaO in neat, loaded and mixed forms, as well as a support for other catalyst systems, CaO reaction
mechanism, CaO tolerance to low to moderate oil qualities and reaction conditions, the conformance of
CaO-catalyzed biodiesel to key specifications and the future outlook and the challenges of the catalyst
are suitably addressed.