This review summarizes the current knowledge on induction, reaction mechanism, occurrence and activity of phenylalanine ammonium-lyase. Attention was paid to defence reactions of plants based on the phenylpropanoid biosynthesis. These diverse compounds include flavonoids, lignin, coumarins and many phenols with a multiplicity of functions in their structural support, pigmentation, defence and signalling. The role of the enzyme as the catalyst of the gate metabolic step from primary metabolism into the phenylpropanoid pathway is also described.