This contribution offers a review, comprehensive to date, of a 15-year research program on the broaden-and-build theory of positive emotions. Although centered on evidence that has emerged from Fredrickson's Positive Emotions and Psychophysiology Laboratory (PEP Lab), it features key findings from other laboratories as well. It begins with a description of 10 representative positive emotions, alongside approaches for assessing them, both directly with the modified Differential Emotions Scale and indirectly through physiological and implicit measures. Next, it offers the seeds of the broaden-and-build theory, including work on the undo effect of positive emotions. It then reviews the state of the evidence for the twin hypotheses that stem from the broaden-and-build theory, the broaden hypothesis and the build hypothesis, including a focus on upward spiral dynamics. It touches next on new frontiers for the theory, including deeper investigations into the biological resources that positive emotions build as well as clinical and organizational applications. Finally, this contribution closes with a brief presentation of two offshoots from the broaden-and-build theory, namely, the upward spiral model of lifestyle change and work on love as positivity resonance between and among people. Both are targets of increasing work in the PEP Lab.