It is known that the equation$ |x - 1| + |x - 2| +... + |x - 2001| = a$ has exactly one solution. Find $a$.
1. **Identify the nature of the function:**
   The given function is \( f(x) = |x - 1| + |x - 2| + \cdots + |x - 2001| \). This function is a sum of absolute values, which is piecewise linear and changes slope at each of the points \( x = 1, 2, \ldots, 2001 \).

2. **Determine the median:**
   The function \( f(x) \) achieves its minimum value when \( x \) is the median of the set \(\{1, 2, \ldots, 2001\}\). Since there are 2001 points, the median is the middle value, which is \( x = 1001 \).

3. **Calculate the value of the function at the median:**
   To find \( a \), we need to evaluate \( f(1001) \):
   \[
   f(1001) = |1001 - 1| + |1001 - 2| + \cdots + |1001 - 2001|
   \]
   This can be split into two sums:
   \[
   f(1001) = \sum_{i=1}^{1000} (1001 - i) + \sum_{i=1002}^{2001} (i - 1001)
   \]

4. **Evaluate the first sum:**
   \[
   \sum_{i=1}^{1000} (1001 - i) = \sum_{i=1}^{1000} 1001 - \sum_{i=1}^{1000} i = 1000 \cdot 1001 - \frac{1000 \cdot 1001}{2} = 1000 \cdot 1001 - 500 \cdot 1001 = 500 \cdot 1001
   \]

5. **Evaluate the second sum:**
   \[
   \sum_{i=1002}^{2001} (i - 1001) = \sum_{i=1}^{1000} i = \frac{1000 \cdot 1001}{2} = 500 \cdot 1001
   \]

6. **Combine the results:**
   \[
   f(1001) = 500 \cdot 1001 + 500 \cdot 1001 = 1000 \cdot 1001 = 1001000
   \]

Conclusion:
\[
a = 1001000
\]

The final answer is \(\boxed{1001000}\).