Find all continuous functions $f: \mathbb R \to \mathbb R$ such that
\[f(x^2-y^2)=f(x)^2 + f(y)^2, \quad \forall x,y \in \mathbb R.\]
1. Given the functional equation:
   \[
   f(x^2 - y^2) = f(x)^2 + f(y)^2 \quad \forall x, y \in \mathbb{R}
   \]
   we start by setting \( y = x \). This gives:
   \[
   f(x^2 - x^2) = f(x)^2 + f(x)^2
   \]
   Simplifying, we get:
   \[
   f(0) = 2f(x)^2
   \]
   Let \( c = f(0) \). Then:
   \[
   c = 2f(x)^2 \quad \forall x \in \mathbb{R}
   \]
   This implies that \( f(x)^2 \) is constant for all \( x \). Let \( f(x) = k \) for some constant \( k \). Then:
   \[
   c = 2k^2
   \]

2. Since \( f \) is continuous and \( f(x) = k \) for all \( x \), \( f \) must be a constant function. We now substitute \( f(x) = k \) back into the original equation to find the possible values of \( k \):
   \[
   f(x^2 - y^2) = k \quad \text{and} \quad f(x)^2 + f(y)^2 = k^2 + k^2 = 2k^2
   \]
   Therefore:
   \[
   k = 2k^2
   \]
   Solving for \( k \), we get:
   \[
   2k^2 - k = 0
   \]
   Factoring out \( k \), we have:
   \[
   k(2k - 1) = 0
   \]
   Thus, the solutions are:
   \[
   k = 0 \quad \text{or} \quad k = \frac{1}{2}
   \]

3. Therefore, the only continuous functions \( f: \mathbb{R} \to \mathbb{R} \) that satisfy the given functional equation are:
   \[
   f(x) = 0 \quad \forall x \in \mathbb{R}
   \]
   and
   \[
   f(x) = \frac{1}{2} \quad \forall x \in \mathbb{R}
   \]

The final answer is \(\boxed{f(x) = 0 \text{ or } f(x) = \frac{1}{2}}\).