To simplify \( 5^{10} \cdot 5^{5} \) using the Product Rule of Exponents, which states that \( a^{m} \cdot a^{n} = a^{m+n} \), we proceed as follows:
\[ 5^{10} \cdot 5^{5} = 5^{10 + 5} = 5^{15} \]
So the simplified expression is \( 5^{15} \).
Therefore, the answer is:
\( 5^{15} \)