Let $M_1, M_2, . . ., M_{11}$ be $5-$element sets such that $M_i \cap M_j \neq {\O}$ for all $i, j \in \{1, . . ., 11\}$. Determine the minimum possible value of the greatest number of the given sets that have nonempty intersection.
1. **Restate the problem in graph theory terms:**
   We need to find the smallest \( n \) such that a complete graph \( K_{11} \) can be covered by some \( K_n \) subgraphs, where each vertex is included in at most 5 of these \( K_n \) subgraphs.

2. **Consider the case \( n = 3 \):**
   - If we cover \( K_{11} \) with triangles (\( K_3 \)), each vertex needs to be adjacent to 10 other vertices.
   - Each triangle a vertex is part of can satisfy 2 of these adjacencies.
   - Therefore, each vertex needs to be in exactly 5 triangles.
   - However, this is impossible because \( 3 \nmid 5 \cdot 11 \). This means that the total number of triangle incidences (55) is not divisible by 3, making it impossible to cover \( K_{11} \) with triangles such that each vertex is in exactly 5 triangles.

3. **Consider the case \( n = 4 \):**
   - We need to check if \( K_{11} \) can be covered by \( K_4 \) subgraphs such that each vertex is included in at most 5 of these \( K_4 \) subgraphs.
   - Each \( K_4 \) has 4 vertices and 6 edges.
   - To cover \( K_{11} \), we need to ensure that each vertex is part of at most 5 \( K_4 \) subgraphs.
   - We need to find a configuration where this is possible.

4. **Construct a configuration for \( n = 4 \):**
   - Consider a \( K_{13} \) graph, which can be covered by \( K_4 \) subgraphs such that each vertex is included in at most 4 of these \( K_4 \) subgraphs.
   - This configuration can be adjusted to fit \( K_{11} \) by removing 2 vertices and ensuring the remaining vertices still satisfy the condition.

5. **Verify the configuration:**
   - By removing 2 vertices from \( K_{13} \) and ensuring the remaining 11 vertices are covered by \( K_4 \) subgraphs, we can achieve the desired coverage.
   - Each vertex in \( K_{11} \) will be part of at most 5 \( K_4 \) subgraphs, satisfying the condition.

Conclusion:
The minimum possible value of the greatest number of the given sets that have a nonempty intersection is \( \boxed{4} \).