The Key European Guidelines for the diagnosis and management of patients with phenylketonuria (PKU) were developed to optimise PKU care. Diet is the cornerstone of treatment, although some patients can benefit from tetrahydrobiopterin (BH4). No intervention is required if the blood phenylalanine concentration is less than 360µmol/l. Treatment is recommended up to the age of 12 years if the phenylalanine blood concentrations is between 360-600µmol/l, and lifelong treatment is recommended if the concentration is more than 600µmol/l. For women trying to conceive and during pregnancy, untreated phenylalanine concentrations of more than 360µmol/l need to be reduced. Treatment target concentrations are as follows: 120-360µmol/l for individuals aged 0-12 years and for maternal PKU, and 120-600µmol/l for non-pregnant individuals older than 12 years