The standard core accretion model of planet formation proceeds from condensation through aggregation of dust particles to formation of km-scale, or larger, “planetesimals”. Once planetesimals form, collisional accretion is aided by the gravity of the planetesimals and simulations consistently produce planets from this starting point. The formation of planetesimals has been problematic, however. Formation by gravitational instability is hampered by turbulence in the disk, and formation by collisional accretion has to occur quickly enough to avoid a fiery fate of the proto-planetesimals in the central star due to aerodynamic drag in the disk. I’ll present the results of experiments designed to study the conditions under which accretion may occur. The experiments also have implications for the collisional evolution of planetary ring systems, and for regolith evolution on planetary satellites and asteroids.