The thickness of a graph   $G$  is the minimum number of planar subgraphs whose union is
 $G$  is the minimum number of planar subgraphs whose union is   $G$ . A
 $G$ . A   $t$ -minimal graph is a graph of thickness
 $t$ -minimal graph is a graph of thickness   $t$  that contains no proper subgraph of thickness
 $t$  that contains no proper subgraph of thickness   $t$ . In this paper, upper and lower bounds are obtained for the thickness,
 $t$ . In this paper, upper and lower bounds are obtained for the thickness,   $t\left( G\,\square \,H \right)$ , of the Cartesian product of two graphs
 $t\left( G\,\square \,H \right)$ , of the Cartesian product of two graphs   $G$  and
 $G$  and   $H$ , in terms of the thickness
 $H$ , in terms of the thickness   $t\left( G \right)$  and
 $t\left( G \right)$  and   $t\left( H \right)$ . Furthermore, the thickness of the Cartesian product of two planar graphs and of a
 $t\left( H \right)$ . Furthermore, the thickness of the Cartesian product of two planar graphs and of a   $t$ -minimal graph and a planar graph are determined. By using a new planar decomposition of the complete bipartite graph
 $t$ -minimal graph and a planar graph are determined. By using a new planar decomposition of the complete bipartite graph   ${{K}_{4k,\,4k}}$ , the thickness of the Cartesian product of two complete bipartite graphs
 ${{K}_{4k,\,4k}}$ , the thickness of the Cartesian product of two complete bipartite graphs   ${{K}_{n,n}}$  and
 ${{K}_{n,n}}$  and   ${{K}_{n,n}}$  is also given for
 ${{K}_{n,n}}$  is also given for   $n\,\ne \,4k\,+\,1$ .
 $n\,\ne \,4k\,+\,1$ .