Nature Reviews Physics is an online-only journal publishing high-quality technical reference, review and commentary articles in all areas of fundamental and applied physics. Nature Reviews Physics is a new online-only reviews journal, publishing monthly from January 2019. Nature Reviews Physics is an exclusively online publication. This enables us to publish review-type articles as soon as they are ready, which benefits authors with an earlier publication date and gives our readers access to accepted papers several weeks before they would appear in a specific issue. Note that papers published online are definitive and may be altered only through the publication of a Corrigendum or Erratum. Therefore, authors should make every effort to ensure that the page proofs are correct.