  Our galaxy is full with planets. We now know that planets and planetary
systems are diverse and come with different sizes, masses and compositions, as
well as various orbital architectures. Although there has been great progress
in understanding planet formation in the last couple of decades, both
observationally and theoretically, several fundamental questions remain
unsolved. This might not be surprising given the complexity of the process that
includes various physical and chemical processes, and spans huge ranges of
length-scales, masses, and timescales. In addition, planet formation cannot be
directly observed but has to be inferred by gluing together different pieces of
information into one consistent picture. "How do planets form?" remains a
fundamental question in modern astrophysics. In this review we list some of the
key open questions in planet formation theory as well as the challenges and
upcoming opportunities.
