If we want to do as much good as possible, we have to compare all the ways of doing good—including ways that involve helping members of different species. But do benefits to humans count the same as benefits to chickens? What about chickens vs. carp? Carp vs. honey bees? In 2020, Rethink Priorities published the Moral Weight Series—a collection of five reports about these and related questions. In May 2021, Rethink Priorities launched the Moral Weight Project, which extended and implemented the research program that those initial reports discussed. This sequence collects Rethink Priorities’ work on cause prioritization across species.