  Over the past two decades, large strides have been made in the field of
planet formation. Yet fundamental questions remain. Here we review our state of
understanding of five fundamental bottlenecks in planet formation. These are:
1) the structure and evolution of protoplanetary disks; 2) the growth of the
first planetesimals; 3) orbital migration driven by interactions between
proto-planets and gaseous disk; 4) the origin of the Solar System's orbital
architecture; and 5) the relationship between observed super-Earths and our own
terrestrial planets. Given our lack of understanding of these issues, even the
most successful formation models remain on shaky ground.
