Abstract: Proof assistants and automated theorem provers generally produce evidence of a successful proof in an assortment of (often ad hoc) formats. Unfortunately, the evidence generated by one proof is seldom useful for another prover or even for a future version of itself. I will outline some recent work on providing a single language for proof certificates for which a simple proof checker can be built and for which a wide range of common proof formats (sequent calculus, natural deduction, tableau, and tabled deduction) can be hosted. A proof checker for proof certificates can then immediately be a proof checker for all these hosted proof systems. Such proof certificates should make it possible for any theorem prover to trust proofs from any other theorem prover and for markets and libraries of proofs to become possible.