The physical barrier between blood and the CNS (the blood-brain barrier, the 
blood-spinal cord barrier and the blood-CSF barrier) protects the CNS from both 
toxic and pathogenic agents in the blood. It is now clear that disruption of the 
blood-CNS barrier plays a key role in a number of CNS disorders, particularly 
those associated with neurodegeneration. Such disruption is inevitably 
accompanied by inflammatory change, as immune cells and immune mediators gain 
access to the brain or spinal cord. The blood-CNS barrier also presents a major 
obstacle for potential CNS medicines. Robust methods to assess CNS permeation 
are therefore essential for CNS drug discovery, particularly when brain 
pharmacokinetics are taken into account and especially when such measures are 
linked to neurochemical, physiological, behavioural or neuroimaging readouts of 
drug action. Drug candidates can be successfully designed to cross the blood-CNS 
barrier, but for those that can't there is the possibility of entry with a 
delivery system that facilitates the movement of drug candidate across the 
blood-CNS barrier.