Primary fatty alcohols are present in all phyla, where they serve various unique biological functions, either in structural materials or biochemical compounds. Fatty alcohols and derivatives provided by living organisms are also important feedstock for the oleochemical and lubricant industries. Since they are biodegradable and generally non-toxic, they can also be used in the manufacture of cosmetics and pharmaceuticals. Fatty alcohols are produced by Fatty Acid Reductases (FAR), which catalyse the reductions of fatty acyl-CoA/ACPs. The reductions are conducted in two consecutive reactions; a fatty acyl-CoA/ACP is first reduced to a fatty aldehyde, which is then further reduced into a fatty alcohol. In prokaryotes, the two-step reduction has until now been considered to be catalysed by two different enzymes, while in eukaryotes it has been found that one enzyme carries out both reduction steps via an intermediate fatty aldehyde, which is thought not to be released during the reductions. This thesis presents data on the in vivo and in vitro characterisation of FARs from Arabidopsis thaliana (AtFARs) and Marinobacter aquaeolei VT8 (Maqu_2220). Contrary to available literature, Maqu_2220 was found to catalyse both steps in the reductions of fatty acyl-CoA/ACPs into fatty alcohols. Thus, at least two biochemical pathways exist among prokaryotes for the reductions of activated fatty acyl chains to fatty alcohols, one of which is the above-identified activity in analogy with eukaryotic FARs. Characterisation of AtFAR6, AtFAR2/MS2 and the bacterial Maqu_2220 revealed that under in vitro conditions, intermediate fatty aldehydes were released and free fatty acids were observed, in addition to fatty alcohols produced as end-products. The in vitro characterisation of AtFAR2/MS2 and AtFAR6 showed that the ratio of fatty alcohol/fatty aldehyde produced by these enzymes depends strongly on chain length and saturation state of the substrates and substrate concentration. Both in vitro and in vivo data consistently showed that the highest activity of AtFAR2/MS2 and AtFAR6 is for the production of C16:0-alcohol from C16:0-CoA/ACP substrates. In a subsequent study, AtFAR6 was found to be a chloroplast-localised FAR enzyme involved in production and accumulation of C16:0-alcohol within this organelle.