The recently updated Japanese guidelines draw attention to a specific MRI pattern of disproportionately enlarged subarachnoid space hydrocephalus (DESH), believed to be pathognomonic of idiopathic normal pressure hydrocephalus (iNPH). This chapter discusses why establishing the diagnosis of NPH remains a challenge fifty years after its classic description. The original diagnosis of NPH relied upon the presence of mild dementia, gait, and urinary difficulties (Hakim's triad) seen in association with ventriculomegaly on pneumo-encephalogram. More sensitive cognitive evaluation of iNPH patients requires specific tests for the assessment of subcortical frontal lobe deficits such as the Rey Auditory Verbal Learning Test, Stroop test, Grooved Pegboard, Trail Making A and B Test, and digit span test. This diagnostic test provides information about cerebrospinal fluid (CSF) dynamics and predicts outcome. It consists in either removal of CSF accompanied by pre and post functional evaluation, or an infusion (bolus or continuous) test.