Over two centuries after the first platypus specimen stirred the scientific community in Europe, the whole-genome sequence of the duck-billed platypus has been completed and is publicly available. After publication of eutherian and marsupial genomes, this is the first genome of a monotreme filling an important evolutionary gap between the divergence of birds more that 300 million years ago and marsupials more than 140 million years ago. Monotremes represent the most basal surviving branch of mammals and the platypus genome sequence allows unprecedented insights into the evolution of mammals and the fascinating biology of the egg-laying mammals. Here, we discuss some of the key findings of the analysis of the platypus genome and point to new findings and future research directions, which illustrate the broad impact of the platypus genome project for understanding monotreme biology and mammalian genome evolution.