  This is a brief review on the work done recently. It is shown that the global
constraints of Gauss' law ensure that the vacuum angle must be quantized in
gauge theories with magnetic monopoles. Our quantization rule is given as
$\theta=0$, or $\theta=2\pi N/n~ (n\neq 0)$ with integer n being the relevant
topological charge of the magnetic monopole and N is an unfixed integer in this
approach. This theoretically confirms further the conclusion originally
proposed by the author$^1$ that the strong CP problem can be solved due to the
existence of magnetic monopoles, the fact that the strong CP-violation can be
only so small or vanishing may imply the existence of magnetic monopoles, and
the universe is open.
