The 3x+1 problem concerns iteration of the map T(n) =(3n+1)/2 if n odd; n/2
if n even. The 3x +1 Conjecture asserts that for every positive integer n>1 the
forward orbit of n includes the integer 1. This paper is an annotated
bibliography of work done on the 3x+1 problem published from 2000 through 2009.
This is a sequel to an annotated bibliography on the 3x+1 problem covering
1963-1999.

At present the 3x+1 Conjecture remains unsolved.