The Buddha, at the center of his teaching, taught "dukkha and the end of dukkha." Yet it is not always clear either what "dukkha" means in this context or what "the end of dukkha" means. In this talk, we explore this core teaching in several ways. First, we distinguish four different meanings of "dukkha" that can be seen in the discourses of the Buddha, only the last of which, interpreted as "reactivity," helps us to make sense of the "end of dukkha." (See the attached PDF file.) This meaning of dukkha can be reconstructed from two core teachings, the "Two Arrows" and Dependent Origination (see the attached PDF file). We then look at several ways of practicing with reactivity, including understanding and working with the common complexity of there frequently being some kind of insight or something important being "mixed" with reactivity, as, for example, when I am very reactive about injustice.