  The 3x+ 1 problem concerns iteration of the map on the integers given by T(n)
= (3n+1)/2 if n is odd; T(n) = n/2 if n is even. The 3x+1 Conjecture asserts
that for every positive integer n > 1 the forward orbit of n under iteration by
T includes the integer 1. This paper is an annotated bibliography of work done
on the 3x+1 problem and related problems from 1963 through 1999. At present the
3x+1 Conjecture remains unsolved.
