Modular forms have a long and important story in the history of mathematics. While we usually think of them as functions on the upper complex half-plane, I will begin by showing how we can also thinking of them as living in the cohomology of interesting subgroups of $\mathrm{SL}(2,\mathbb{Z})$. (I will also say what ``cohomology'' is.) This leads to the notion of ``modular forms'' for groups like $\mathrm{SL}(2,\mathbb{Z}[i])$, and while for $\mathrm{SL}(2,\mathbb{Z})$, everything ``lives over $\mathbb{C}$,'' there are characteristic $p$ phenomenon for imaginary quadratic fields. This relates to recent work of Scholze, which I may try to explain in this special case.