  We discuss the results of laboratory measurements and theoretical models
concerning the aggregation of dust in protoplanetary disks, as the initial step
toward planet formation. Small particles easily stick when they collide and
form aggregates with an open, often fractal structure, depending on the growth
process. Larger particles are still expected to grow at collision velocities of
about 1m/s. Experiments also show that, after an intermezzo of destructive
velocities, high collision velocities above 10m/s on porous materials again
lead to net growth of the target. Considerations of dust-gas interactions show
that collision velocities for particles not too different in surface-to-mass
ratio remain limited up to sizes about 1m, and growth seems to be guaranteed to
reach these sizes quickly and easily. For meter sizes, coupling to nebula
turbulence makes destructive processes more likely. Global aggregation models
show that in a turbulent nebula, small particles are swept up too fast to be
consistent with observations of disks. An extended phase may therefore exist in
the nebula during which the small particle component is kept alive through
collisions driven by turbulence which frustrates growth to planetesimals until
conditions are more favorable for one or more reasons.
