Find all polynomials $f(x)$ with real coefficients having the property $f(g(x))=g(f(x))$ for every polynomial $g(x)$ with real coefficients.
1. **Given Condition**: We need to find all polynomials \( f(x) \) with real coefficients such that \( f(g(x)) = g(f(x)) \) for every polynomial \( g(x) \) with real coefficients.

2. **Initial Insight**: Consider the polynomial \( g(x) = x + 1 \). Substituting \( g(x) \) into the given condition, we get:
   \[
   f(x + 1) = g(f(x)) = f(x) + 1
   \]
   This implies that \( f(x) \) must satisfy the functional equation \( f(x + 1) = f(x) + 1 \).

3. **Functional Equation Analysis**: Let's assume \( f(x) \) is a polynomial of degree \( n \). Write \( f(x) \) in the general form:
   \[
   f(x) = a_n x^n + a_{n-1} x^{n-1} + \cdots + a_1 x + a_0
   \]
   Applying the functional equation \( f(x + 1) = f(x) + 1 \), we get:
   \[
   a_n (x + 1)^n + a_{n-1} (x + 1)^{n-1} + \cdots + a_1 (x + 1) + a_0 = a_n x^n + a_{n-1} x^{n-1} + \cdots + a_1 x + a_0 + 1
   \]

4. **Degree Comparison**: The left-hand side expands to:
   \[
   a_n (x^n + nx^{n-1} + \cdots) + a_{n-1} (x^{n-1} + (n-1)x^{n-2} + \cdots) + \cdots + a_1 (x + 1) + a_0
   \]
   Comparing the highest degree terms on both sides, we see that the degree of \( f(x) \) must be 1 because the highest degree term on the right-hand side is \( a_n x^n \), and on the left-hand side, it is also \( a_n x^n \). For the equality to hold for all \( x \), \( n \) must be 1.

5. **Linear Polynomial**: Since \( f(x) \) is a polynomial of degree 1, let:
   \[
   f(x) = ax + b
   \]
   Substituting into the functional equation \( f(x + 1) = f(x) + 1 \), we get:
   \[
   a(x + 1) + b = ax + b + 1
   \]
   Simplifying, we find:
   \[
   ax + a + b = ax + b + 1 \implies a = 1
   \]
   Therefore, \( f(x) = x + b \).

6. **Checking the Solution**: We need to check if \( f(x) = x + b \) satisfies the original condition \( f(g(x)) = g(f(x)) \) for every polynomial \( g(x) \). Substituting \( f(x) = x + b \), we get:
   \[
   f(g(x)) = g(x) + b \quad \text{and} \quad g(f(x)) = g(x + b)
   \]
   For these to be equal for all \( g(x) \), \( b \) must be 0. Hence, \( f(x) = x \) is the only solution.

\(\blacksquare\)

The final answer is \( \boxed{ f(x) = x } \)