Understanding the structure and function of cortical circuits requires the 
identification of and control over specific cell types in the cortex. To address 
these obstacles, recent optogenetic approaches have been developed. The capacity 
to activate, silence, or monitor specific cell types by combining genetics, 
virology, and optics will decipher the role of specific groups of neurons within 
circuits with a spatiotemporal resolution that overcomes standard approaches. In 
this review, the various strategies for selective genetic targeting of a defined 
neuronal population are discussed as well as the pros and cons of the use of 
transgenic animals and recombinant viral vectors for the expression of 
transgenes in a specific set of neurons.