To simplify the expression \( 3^8 \cdot 3^3 \) using the Product Rule of Exponents, we use the property that states:
\[ a^m \cdot a^n = a^{m+n} \]
In this case, \( a = 3 \), \( m = 8 \), and \( n = 3 \). Applying the Product Rule:
\[ 3^8 \cdot 3^3 = 3^{8+3} = 3^{11} \]
Thus, the simplified expression is:
\[ 3^{11} \]