  After 25 years of laboratory research on protoplanetary dust agglomeration, a
consistent picture of the various processes that involve colliding dust
aggregates has emerged. Besides sticking, bouncing and fragmentation, other
effects, like, e.g., erosion or mass transfer, have now been extensively
studied. Coagulation simulations consistently show that $\rm \mu$m-sized dust
grains can grow to mm- to cm-sized aggregates before they encounter the
bouncing barrier, whereas sub-$\rm \mu$m-sized water-ice particles can directly
grow to planetesimal sizes. For siliceous materials, other processes have to be
responsible for turning the dust aggregates into planetesimals. In this
article, these processes are discussed, the physical properties of the emerging
dusty or icy planetesimals are presented and compared to empirical evidence
from within and without the Solar System. In conclusion, the formation of
planetesimals by a gravitational collapse of dust "pebbles" seems the most
likely.
