Since their introduction, the selective serotonin reuptake inhibitors (SSRIs) have become one of the most widely used classes of medication in psychiatry. Their popularity is based on apparent efficacy over a wide range of disorders and a favorable side-effect profile. However, as with any psychotropic medication, considerable data are required to define where a drug works and where it does not. There is now a wealth of evidence demonstrating that SSRIs may differ in their efficacy profiles in certain depressive symptoms, in different subtypes of depression, with respect to their ability to maintain efficacy over time, on broader outcomes such as quality of life, and in the consistency of the usually effective minimum therapeutic dose across the age spectrum and across indications. Although this review includes data on all SSRIs, it focuses on fluoxetine and sertraline, which in addition to being the most widely used SSRIs are also the most widely studied. The relative quantity and quality of data on these two SSRIs means that it is possible to make relatively firm inferences regarding their differential effects on affective symptoms and quality of life.