Physiological sensing has been a prominent fixture in games user research (GUR) since the late 1990s, when researchers started to explore its potential to enhance and understand experience within digital game play. Since these early days, is has been widely argued that "affective gaming"—in which gameplay is influenced by a player’s emotional state—can enhance player experience by integrating physiological sensors into play. Reflecting in 2020 on these last 20 years of progress, we systematically review the field of affective gaming to answer whether or not it has met this promise. We create a framework (the affective game loop) to classify 162 publications (included from an initial search set of 52,834 results) within affective gaming, present findings on the current state of the field, and discuss how our findings, contextualized within the affective game loop, can guide the future of affective gaming.