The tail-suspension test (TST) is a widely used assay for screening potential antidepressant drugs. Its advantages include being a rapid, inexpensive, highly predictive and high-throughput screening test for the acute behavioral effects of antidepressants. The test is based on the principle that mice subjected to the short-term, inescapable stress of being suspended by their tail, will develop an immobile posture. A variety of antidepressant treatments irrespective of their primary mechanism of action decrease the time mice spend immobile by promoting escape-oriented behaviors. In recent years, the TST has been used for the behavioral characterization of genetically modified mice in an effort to identify novel targets of antidepressant activity for which pharmacological tools may not yet exist. Additionally, such mice are also tested in the TST for in vivo evaluation of the validity of current molecular theories of depression and antidepressant action. In this chapter, we provide a historical overview of the TST and discuss the utility and the validity of the TST as an animal model of antidepressant activity. This is followed by a detailed practical protocol on how to conduct a TST experiment in the laboratory as well as some troubleshooting tips. We describe important experimental variables that can interfere with the outcome of a TST experiment, and discuss how some of these factors have been manipulated in an effort to identify neural substrates that contribute to both baseline and antidepressant-induced changes in immobility. Finally, we also provide some examples of how such findings have been extrapolated to clinical depression. We conclude that the TST has great utility as a simple model to rapidly evaluate antidepressant-induced behavior and under some circumstances may also prove useful in determining genetic contributions to depression.