  For half a century, the Roper resonance has defied understanding. Discovered
in 1963, it appears to be an exact copy of the proton except that its mass is
50% greater. The mass is the first problem: it is difficult to explain with any
theoretical tool that can validly be used to study quantum chromodynamics
[QCD]. In the last decade, a new challenge has appeared, viz. precise
information on the proton-to-Roper electroproduction transition form factors,
reaching $Q^2\approx 4.5\,$GeV$^2$. This scale probes the domain within which
hard valence-quark degrees-of-freedom could be expected to determine form
factor behavior. Hence, with this new data the Roper resonance becomes a
problem for strong-QCD [sQCD]. An explanation of how and where the Roper
resonance fits into the emerging spectrum of hadrons cannot rest on a
description of its mass alone. Instead, it must combine this with a detailed
understanding of the Roper's structure and how that is revealed in the
transition form factors. Furthermore, it must unify all this with a similarly
complete picture of the proton. This is a prodigious task, but a ten-year
international effort, drawing together experimentalists and theorists, has
presented a solution to the puzzle. Namely, the Roper is at heart the proton's
first radial excitation, consisting of a dressed-quark core augmented by a
meson cloud that reduces the core mass by approximately 20% and materially
alters its electroproduction form factors on $Q^2<2m_N^2$, where $m_N$ is the
proton's mass. We describe the experimental motivations and developments which
enabled electroproduction data to be procured within a domain that is
unambiguously the purview of sQCD, thereby providing a real challenge and
opportunity for modern theory; and survey the developments in reaction models
and QCD theory that have enabled this conclusion to be drawn about the nature
of the Roper resonance.
