In his Antwerp paper on p-adic modular forms, Serre remarks by way of introduction that congruences among families of Bernoulli numbers are also satisfied by families of Eisenstein series. I will report on the work of a number of people, particularly Hopkins, Goerss, and Rezk, showing that the Atiyah-Bott-Shapiro orientation and the string orientation of tmf are instances of this principle.