 Trials of cholinesterase inhibitors in PSP have produced rather conflicting results: donepezil improves cognition but deteriorates some motor functions whereas a case series of 5 PSP patients treated with rivastigmine found an improvement in several cognitive aspects and no deterioration of motor functions .On the other hand in Parkinsons disease there is convincing evidence of a positive effect of rivastigmine on cognition , apathy and falls Investigators hypothesis is that rivastigmine an acetyl- and butyryl-cholinesterase inhibitor may reduce gait and postural impairment in PSP and may therefore limit the number of falls and their consequences both in terms of injuries sustained fractures etc... and on the patients autonomy. In addition investigators hypothesise that rivastigmine may also reduce the cognitive and behavioural impairment associated with PSP. Taken together these improvements are likely to produce a significant effect on the patients quality of life and their caregiver burden. 