Optogenetic tools comprise a variety of different light-sensitive proteins from 
single-cell organisms that can be expressed in mammalian neurons and effectively 
control their excitability. Two main classes of optogenetic tools allow to 
either depolarize or hyperpolarize, and respectively generate or inhibit action 
potentials in selective populations of neurons. This opens unprecedented 
possibilities for delineating the role of certain neuronal populations in brain 
processing and diseases. Moreover, optogenetics may be considered for developing 
potential treatment strategies for brain diseases, particularly for excitability 
disorders such as epilepsy. Expression of the inhibitory halorhodopsin NpHR in 
hippocampal principal cells has been recently used as a tool to effectively 
control chemically and electrically induced epileptiform activity in slice 
preparations, and to reduce in vivo spiking induced by tetanus toxin injection 
in the motor cortex. In this review we give a comprehensive summary of what has 
been achieved so far in the field of epilepsy using optogenetics, and discuss 
some of the possible strategies that could be envisaged in the future. We also 
point out some of the challenges and pitfalls in relation to possible outcomes 
of using optogenetics for controlling network excitability, and associated brain 
diseases. This article is part of the Special Issue entitled 'New Targets and 
Approaches to the Treatment of Epilepsy'.