  One of the most important developments in exoplanet science in the past
decade is the discovery of multi-planet systems with sub-Neptune-sized planets
interior to 1~AU. This chapter explores the architectures of these planetary
systems, which often display a remarkable degree of uniformity: the planets
have nearly equal sizes, regular orbital spacing, low eccentricities, and small
mutual inclinations. This uniformity stands in sharp contrast to the diverse
nature of the exoplanet sample considered as a whole (as well as our inner
solar system). We begin with a critical review of the observations -- including
possible biases -- and find that these peas-in-a-pod planetary systems are
apparently a common outcome of the planet formation process. Modest departures
from exact uniformity suggest additional patterns, such as the planet mass
slowly increasing with semi-major axis. The star formation process naturally
produces circumstellar disks with the properties required to produce these
planetary systems, although the solid material must move inward from its
initial location. We discuss primary modes of planetary assembly, the role of
orbital migration, and post-nebular atmospheric loss. Mature planetary systems
are found to be near their minimum energy (tidal equilibrium) configurations;
this finding provides a partial explanation for their observed properties and
indicates that efficient energy dissipation must occur. Finally, we consider
population synthesis models and show that peas-in-a-pod patterns emerge with
reasonable choices for the input parameters. Nonetheless, interesting
observational and theoretical challenges remain in order to understand how
these surprisingly organized planetary systems arise from the disorder of their
formation processes.
