  Two members of a packing are neighbors if they have a common boundary point.
A multitude of problems arises in connection with neighbors in a packing. The
oldest one concerns a dispute between Newton and Gregory about the maximum
number of neighbors a member can have in a packing of congruent balls. Other
problems ask for the average number of neighbors or the maximum number of
mutually neighboring members in a packing. The present work gives a survey of
these problems.
