This chapter discusses critical analysis of the clinical findings in Parkinson's disease (PD) patients grafted with embryonic mesencephalic tissue. The emphasis is on the magnitude, pattern and time course of clinical recovery and the relation between the graft-induced symptomatic relief and the restoration of striatal dopaminergic neurotransmission. In addition, the chapter discusses some methodological issues and different strategies to improve the functional recovery following neural transplantation. The clinical observations after neural transplantation clearly documents that cell replacement has the potential to become a novel and effective therapeutic strategy for large numbers of patients with PD. However, neural transplantation currently remains an experimental approach and can only be applied to small groups of patients. The urgent problem is to develop the new sources of dopaminergic cells that are available in large amounts and suitable for transplantation in humans. It is conceivable that these cells, to give rise to maximal symptomatic relief, must exhibit several of the properties characteristic of human embryonic mesencephalic dopamine neurons, such as formation of specialized synaptic contacts with host neurons, regulated dopamine release, and functional integration into the patient's brain.