  Deepening our knowledge of the partonic content of nucleons and nuclei
represents a central endeavour of modern high-energy and nuclear physics, with
ramifications in related disciplines such as astroparticle physics. There are
two main scientific drivers motivating these investigations of the partonic
structure of hadrons. On the one hand, addressing fundamental open issues in
our understanding in the strong interactions such as the origin of the nucleon
mass, spin, and transverse structure; the presence of heavy quarks in the
nucleon wave function; and the possible onset of novel gluon-dominated
dynamical regimes. On the other hand, pinning down with the highest possible
precision the substructure of nucleons and nuclei is a central component for
theoretical predictions in a wide range of experiments, from proton and heavy
ion collisions at the Large Hadron Collider to ultra-high energy neutrino
interactions at neutrino telescopes. This Article presents a succinct
non-technical overview of our modern understanding of the quark, gluon, and
photon substructure of nucleons and nuclei, focusing on recent trends and
results and discussing future perspectives for the field.
