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One of the most important developments in exoplanet science in the pastdecade is the discovery of multi-planet systems with sub-Neptune-sized planetsinterior to 1~AU. This chapter explores the architectures of these planetarysystems, which often display a remarkable degree of uniformity: the planetshave nearly equal sizes, regular orbital spacing, low eccentricities, and smallmutual inclinations. This uniformity stands in sharp contrast to the diversenature of the exoplanet sample considered as a whole (as well as our innersolar system). We begin with a critical review of the observations -- includingpossible biases -- and find that these peas-in-a-pod planetary systems areapparently a common outcome of the planet formation process. Modest departuresfrom exact uniformity suggest additional patterns, such as the planet massslowly increasing with semi-major axis. The star formation process naturallyproduces circumstellar disks with the properties required to produce theseplanetary systems, although the solid material must move inward from itsinitial location. We discuss primary modes of planetary assembly, the role oforbital migration, and post-nebular atmospheric loss. Mature planetary systemsare found to be near their minimum energy (tidal equilibrium) configurations;this finding provides a partial explanation for their observed properties andindicates that efficient energy dissipation must occur. Finally, we considerpopulation synthesis models and show that peas-in-a-pod patterns emerge withreasonable choices for the input parameters. Nonetheless, interestingobservational and theoretical challenges remain in order to understand howthese surprisingly organized planetary systems arise from the disorder of theirformation processes.

Architectures of Compact Multi-planet Systems: Diversity and Uniformity