This chapter discusses functional neurosurgery designed to modulate dysfunctional neural circuits mediating sensorimotor, cognitive, emotional, and volitional capacities. The chapter assesses the comparative benefits and risks of neural ablation and deep brain stimulation as the two most invasive forms of neuromodulation. It discusses the question of whether individuals with a severe or moderately severe psychiatric disorder have enough cognitive and emotional capacity to weigh reasons for and against ablation or deep brain stimulation and give informed consent to undergo it. The chapter also discusses the obligations of investigators conducting these trials to research subjects. In addition, it examines the medical and ethical justification for a sham control arm in psychiatric neurosurgery clinical trials. It considers the therapeutic potential of optogenetics as a novel form of neuromodulation. The fact that this technique manipulates both genetic material and neural circuits and has been tested only in animal models makes it unclear what its benefit–risk ratio would be. The chapter concludes with a brief discussion of the potential of neuromodulation to stimulate endogenous repair and growth mechanisms in the brain.