i will introduce a program, begun by dan-cohen and wewers, to compute minhyong kim's selmer varieties using mixed tate motives. the idea is as follows. we care about the galois action on the unipotent fundamental group of $\mathbf{p}^1 \setminus \{0,1,\infty\}$. this galois action lives in a certain category of $p$-adic galois representations known as mixed tate representations. we will see that this category is tannakian and has a fairly simple description, in terms of its ext groups, which are just bloch-kato selmer groups. the bloch-kato selmer groups are $p$-adic vector spaces, but they also have a rational structure coming from algebraic k-theory. the category of mixed tate motives gives us a $\mathbb{q}$-linear tannakian category that underlies the $\mathbb{q}_p$-linear tannakian category of galois representations. this in turn allows us to define a selmer variety over $\mathbb{q}$ (rather than $\mathbb{q}_p$), and a more explicit understanding of the category of mixed tate motives allows us to compute this variety explicitly.