Step 1: Recall the condition for strict increase.
A function \( f(x) \) is strictly increasing on an interval if its derivative is positive on that interval. That is,
\[
f'(x) > 0 \text{for all } x \in \mathbb{R}.
\]
Step 2: State the condition.
Thus, for \( f(x) \) to be strictly increasing for all \( x \in \mathbb{R} \), the condition is:
\[
f'(x) > 0 \text{for all } x \in \mathbb{R}.
\]
Final Answer: \[ \boxed{f'(x) > 0 \text{ for all } x \in \mathbb{R}} \]