Determine the set of all polynomials $P(x)$ with real coefficients such that the set $\{P(n) | n \in \mathbb{Z}\}$ contains all integers, except possibly finitely many of them.
1. **Understanding the problem**: We need to find all polynomials \( P(x) \) with real coefficients such that the set \( \{P(n) \mid n \in \mathbb{Z}\} \) contains all integers, except possibly finitely many of them. This means that for sufficiently large \( x \), every integer can be expressed as \( P(n) \) for some integer \( n \).

2. **Degree of the polynomial**: Consider the degree of the polynomial \( P(x) \). If \( P(x) \) has a degree greater than 1, then the difference \( P(x+1) - P(x) \) will grow without bound as \( x \) increases. This is because the leading term of \( P(x) \) will dominate, making the difference arbitrarily large. Therefore, \( P(x) \) must be of degree at most 1.

3. **Form of the polynomial**: Since \( P(x) \) is of degree at most 1, it can be written as \( P(x) = ax + b \) where \( a \) and \( b \) are real numbers. 

4. **Constraints on \( a \)**: For \( P(x) = ax + b \) to cover all integers except possibly finitely many, \( a \) must be rational. If \( a \) were irrational, then \( ax + b \) would not be able to hit all integers for integer \( x \). Let \( a = \frac{p}{r} \) where \( p \) and \( r \) are integers with \( r > 0 \).

5. **Ensuring all integers are covered**: For \( P(x) = \frac{p}{r}x + b \) to cover all integers except possibly finitely many, \( \frac{p}{r} \) must be such that \( P(x) \) can take on all integer values. This implies that \( p \le r \), otherwise, the gaps between consecutive values of \( P(x) \) would be too large, missing infinitely many integers.

6. **Form of \( b \)**: Since \( P(x) \) must map integers to integers, \( b \) must also be rational. Let \( b = \frac{q}{r} \) where \( q \) is an integer.

7. **Final form of the polynomial**: Combining these, we get \( P(x) = \frac{p}{r}x + \frac{q}{r} \). For \( P(x) \) to map integers to integers, \( r \) must divide \( q \). Let \( q = kr \) for some integer \( k \). Thus, \( P(x) = \frac{p}{r}x + k \).

8. **Simplifying the polynomial**: We can rewrite \( P(x) \) as \( P(x) = \frac{p}{r}(x + \frac{kr}{p}) \). For \( P(x) \) to be an integer for all integers \( x \), \( \frac{kr}{p} \) must be an integer, implying \( p \) divides \( kr \). Since \( p \le r \), we can simplify this to \( P(x) = \frac{x + c}{k} \) where \( c \) is an integer and \( k \) is a positive integer.

Therefore, the only polynomials that satisfy the given condition are of the form \( P(x) = \frac{x + c}{k} \) where \( c \) is an integer and \( k \) is a positive integer.

The final answer is \( \boxed{ P(x) = \frac{x + c}{k} } \) where \( c \) is an integer and \( k \) is a positive integer.