In recent years, three inherited defects in the biosynthesis and transport of creatine have been described. The biosynthetic defects include deficiencies of l-arginine:glycine amidinotransferase and guanidinoacetate methyltransferase. The third defect is a functional defect in the creatine transporter (SLC6A8). Clinical symptoms of the three defects vary in severity, are aspecific and include mental retardation with severe speech delay, autistiform behaviour, and epilepsy. Some patients with GAMT deficiency exhibit a more complex clinical phenotype with extrapyramidal movement disorder. All three defects can be diagnosed by in vivo proton magnetic resonance spectroscopy of the brain, which shows a severe reduction or absence of creatine. Laboratory investigations for the diagnosis start with the analysis of guanidinoacetate, creatine and creatinine in body fluids (plasma and urine). Based on these findings, enzyme assays for AGAT or GAMT, or a creatine uptake assay for the transporter defect can be performed. DNA mutation analysis of the genes involved can prove the defects at the molecular level. To diagnose female patients with SLC6A8 deficiency, mutation analysis may be the only choice.