  There is a positive constant $c_1$ such that for any diagram $D$ representing
the unknot, there is a sequence of at most $2^{c_1 n}$ Reidemeister moves that
will convert it to a trivial knot diagram, $n$ is the number of crossings in
$D$. A similar result holds for elementary moves on a polygonal knot $K$
embedded in the 1-skeleton of the interior of a compact, orientable,
triangulated $PL$ 3-manifold $M$. There is a positive constant $c_2$ such that
for each $t \geq 1$, if $M$ consists of $t$ tetrahedra, and $K$ is unknotted,
then there is a sequence of at most $2^{c_2 t}$ elementary moves in $M$ which
transforms $K$ to a triangle contained inside one tetrahedron of $M$. We obtain
explicit values for $c_1$ and $c_2$.
