Theories of the structure of affect make competing predictions about whether people can feel happy and sad at the same time. Considerable evidence that happiness and sadness can co-occur has accumulated in the past 15 years, but holes in the case remain. I describe those holes and suggest strategies for testing them in future research. I also explore the possibility that the case may never be closed, in part because the competing hypotheses may not be entirely falsifiable. Fortunately, hypotheses need (...) not be falsifiable to be useful. Research on mixed emotions has been generative and the body of research will continue to shed light on the structure of affect. (shrink)