Understanding the relationship between neuronal responses and behavior is a key step in the functional dissection of neural circuits.Traditional electrophysiology provides the highest level of temporal accuracy,but falls short in the total recorded cell number and cell-type specificity.Although the electrophysiology with "optical tagging" provides cell-type information,this approach is technically challenging and often limited by low tagging efficiency.As an alternative,fiber photometry provides the most sensitive and easiest way to record cell-type specific neuronal activities of the deep brain structures in behaving animals.In this talk,I will introduce the working mechanism of fiber photometry,and show some examples to illustrate the application of this technique in the recording of genetically identified neurons in dorsal raphe nuclei (DRN).