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Abstract

The canonical model for the formation of terrestrial planets and giant planet cores implicitly relies on an early and efficient phase of planetesimal growth in a gas-rich circumstellar disk. But, as theorists have known for decades now, there are some formidable obstacles to meeting that requirement. Many of these problems, and potentially their solutions, are associated with the growth and migration of "pebbles" (̃mm/cm-sized solids) in the first few million years of a disk's lifetime. That is especially fortuitous, since the thermal continuum emission from these particles in nearby disks can be readily detected and resolved with long-baseline radio interferometers. This chapter describes what is being learned about the early evolution of solids by comparing such data with sophisticated simulations. Specifically, the focus will be on the observable signatures of particle growth and migration and the mounting evidence that small-scale substructures in the (gas) disk play fundamental - and perhaps mandatory - roles in the planet formation process.