Abstract: Planetesimals are small rocky (and sometimes icy) objects, typically 1-100 km in diameter (e.g., asteroids and comets in the Solar System). One of the largest unanswered questions in all of planetary astrophysics is: how do these planetesimals form in the disks that orbit young, newly forming stars (“circumstellar disks”)? In this talk, I will discuss my group’s research projects devoted to answering this important question. I will first provide an overview of planet formation and describe how planetesimals are an integral step in the planet formation process. I will then discuss the progress my group has made in understanding planetesimal formation with theoretical and computational models. In particular, by using computational fluid and particle dynamics, run on large-scale supercomputing facilities, we are working towards understanding under what conditions and in what locations planetesimals can or cannot form in circumstellar disks. I will conclude with a set of future goals to connect what we have learned so far to the larger picture of planet formation.