Let there be two circles. Find all points $M$ such that there exist two points, one on each circle such that $M$ is their midpoint.
1. Let the centers of the two circles be \( O_1 \) and \( O_2 \) with radii \( r_1 \) and \( r_2 \) respectively. Let \( N \) be the midpoint of the segment \( O_1O_2 \).

2. Consider a point \( A \) on the first circle \((O_1)\) and a point \( B \) on the second circle \((O_2)\). We need to find the locus of the midpoint \( M \) of the segment \( AB \).

3. Let \( \overrightarrow{O_1A} \) and \( \overrightarrow{O_2B} \) be the position vectors of points \( A \) and \( B \) with respect to \( O_1 \) and \( O_2 \) respectively. Then, the position vector of \( M \) with respect to \( N \) is given by:
   \[
   \overrightarrow{NM} = \frac{1}{2} (\overrightarrow{O_1A} + \overrightarrow{O_2B})
   \]

4. The maximum length of \( NM \) occurs when \( \overrightarrow{O_1A} \) and \( \overrightarrow{O_2B} \) are parallel and in the same direction. In this case, the length of \( NM \) is:
   \[
   NM_{\text{max}} = \frac{1}{2} (r_1 + r_2)
   \]

5. The minimum length of \( NM \) occurs when \( \overrightarrow{O_1A} \) and \( \overrightarrow{O_2B} \) are parallel but in opposite directions. In this case, the length of \( NM \) is:
   \[
   NM_{\text{min}} = \frac{1}{2} |r_1 - r_2|
   \]

6. Therefore, the locus of \( M \) is a circle centered at \( N \) with radius ranging from \( \frac{1}{2} |r_1 - r_2| \) to \( \frac{1}{2} (r_1 + r_2) \).

Conclusion:
The locus of \( M \) is the annular region centered at \( N \) with inner radius \( \frac{1}{2} |r_1 - r_2| \) and outer radius \( \frac{1}{2} (r_1 + r_2) \).

The final answer is the annular region centered at \( \boxed{ N } \) with inner radius \( \frac{1}{2} |r_1 - r_2| \) and outer radius \( \frac{1}{2} (r_1 + r_2) \).