To simplify the expression \( 3^8 \cdot 3^3 \) using the Product Rule of Exponents, you can add the exponents when multiplying the same base. The Product Rule states that:
\[ a^m \cdot a^n = a^{m+n} \]
In this case, the base is \( 3 \), and the exponents are \( 8 \) and \( 3 \).
Applying the Product Rule:
\[ 3^8 \cdot 3^3 = 3^{8+3} = 3^{11} \]
Therefore, the simplified expression is:
\[ \boxed{3^{11}} \]