  Recent experimental and theoretical advancements have led to significant
progress in our understanding of the electromagnetic structure of nucleons
($N$), nucleon excitations ($N^\ast$), and other baryons. These breakthroughs
have been made possible by the capabilities of modern facilities, enabling the
induction of photo- and electro-excitation of nucleon resonances. Recent
experimental advances have sparked notable developments in theoretical
approaches. New theoretical methods have been tested and proven to be robust,
marking the beginning of a new era in our understanding on baryons. We present
a comprehensive review of progress in experimental data on $\gamma^\ast N \to
N^\ast$ reactions. Additionally, we discuss various analyses and theoretical
results. Some of these methods have matured in their predictive power, offering
new perspectives on exotic hadrons with multiquark components. We place special
emphasis on both the low-$Q^2$ and large-$Q^2$ regions to reinforce crucial
physical constraints on observables that hold in these limits. Furthermore, we
illustrate that the combination of lattice QCD with chiral effective field
theory and quark models, respectively, proves beneficial in interpreting data
and applying constraints within those different regimes. As a practical
contribution and for future reference, we review the formulas for helicity
amplitudes, multipole form factors and the relations between these two sets of
functions for transitions to resonances with general spin $J \geq \frac{1}{2}$.
These formulas are ubiquitous and play a pivotal role in experimental and
theoretical studies on baryon structure. Notably, the multipole transition form
factors for $J \ge \frac{3}{2}$ resonances serve as valuable tools to test
perturbative QCD results in the large-$Q^2$ region, thanks to the correlations
between electric and magnetic transition form factors.
