It is 100 years since Ernest Rutherford published his results proving the existence of the proton. For many decades the proton was considered elementary. But ever since experiments at SLAC and DESY started firing electrons into protons, beginning in the 1960s, deep-inelastic-scattering experiments have revealed a complex internal picture.
In this issue we take an expert tour of physicists' evolving understanding of the proton, and find that there is still much to learn about this ubiquitous particle - including the origin of its spin, whether or not it decays and the puzzling value of its radius. Flavour physics is another theme of the issue. LHCb's observation of CP violation in the charm sector represents a milestone result, and the collaboration recently released an update of the ratio R K concerning the ratio of certain B-meson decays. From a theoretical perspective, new gauge bosons and leptoquarks are promising potential explanations for the current anomalies reported in the b-quark system, although the picture is far from clear and more data are needed. Meanwhile, researchers are also searching for ultra-rare muon decays that violate lepton-number conservation. Also in this issue: LHCb's discovery of a new pentaquark, DESY's astroparticle ambitions, news on the International Linear Collider, the first image of the centre of a galaxy, and more.