CLN2 disease is an inherited metabolic storage disorder caused by the deficiency of the lysosomal enzyme tripeptidyl peptidase 1 (TPP1). The disease affects mainly the brain and the retina and is characterized by progressive dysfunction of the central nervous system, leading to dementia, epilepsy, loss of motor function and blindness. The classical late infantile type begins at around three years of age with epilepsy and/or a standstill of psychomotor development, followed by a rapid loss of all abilities and death in childhood. A late onset form in a small proportion of patients starts at the age of 4 to 10 years, but also leads to severe neurological deterioration. The deficiency of TPP1 causes the lysosomal accumulation of a material called ceroid lipofuscin. The natural substrate of TPP1 is not known, nor is the connection between storage process and neurodegeneration, which is characterized by loss of neurons. Among various experimental approaches to treatment, enzyme replacement therapy (ERT) and gene therapy have developed remarkably. Enzyme delivery through the cerebrospinal fluid led to wide distribution of enzyme activity in the brain and to attenuated neuropathology and disease progression in a TPP1-deficient mouse model as well as in a natural TPP1-deficient dog model. Safety of the intrathecal delivery, pharmacokinetics, and tissue distribution of the administered enzyme studied in non-human primates were encouraging, and a phase I/II clinical trial for intraventricular ERT in CLN2 patients is ongoing. A second approach uses intracerebral injection of viral vectors containing normal coding segments of the CLN2 gene. In a CLN2 mouse model, this procedure resulted in cerebral enzyme expression, reduced brain pathology and increased survival. A small number of patients have been treated the same way using an AAV2-vector for gene transfer to the brain. Although there were no serious adverse events unequivocally attributable to the vector used, there were some serious adverse effects, and a clinical benefit was not clearly evident under the conditions of the experiment. A phase I/phase II study using a AAVrh10 vector is presently recruiting patients.