Let $ABCD$ be a convex quadrilateral. Determine all points $M$, which lie inside $ABCD$, such that the areas of $ABCM$ and $AMCD$ are equal.
1. Let \(ABCD\) be a convex quadrilateral and \(M\) be a point inside \(ABCD\) such that the areas of \(ABCM\) and \(AMCD\) are equal. We need to find all such points \(M\).

2. Consider the diagonal \(AC\) of the quadrilateral \(ABCD\). We will analyze the position of \(M\) with respect to this diagonal.

3. **Case 1**: \(M\) lies on the half-plane of \(AC\) that contains \(D\).

   - Let \(X\) be the reflection of \(B\) over \(M\). Since \(M\) is the midpoint of \(BX\), we have \(MB = MX\).

   - The area of \(ABCM\) can be expressed as:
     \[
     A_{ABCM} = A_{ABM} + A_{BCM}
     \]
     Similarly, the area of \(AMCD\) can be expressed as:
     \[
     A_{AMCD} = A_{AMC} + A_{MCD}
     \]

   - Given that \(A_{ABCM} = A_{AMCD}\), we have:
     \[
     A_{ABM} + A_{BCM} = A_{AMC} + A_{MCD}
     \]

   - Since \(X\) is the reflection of \(B\) over \(M\), the triangles \(ABM\) and \(AXM\) are congruent, and so are the triangles \(BCM\) and \(CXM\). Therefore:
     \[
     A_{ABM} = A_{AXM} \quad \text{and} \quad A_{BCM} = A_{CXM}
     \]

   - Thus, we can rewrite the area equality as:
     \[
     A_{AXM} + A_{CXM} = A_{AMC} + A_{MCD}
     \]

   - Since \(A_{AXM} + A_{CXM} = A_{ACX}\), we have:
     \[
     A_{ACX} = A_{AMC} + A_{MCD}
     \]

   - This implies that the area of \(ADC\) is equal to the area of \(ACX\):
     \[
     A_{ADC} = A_{ACX}
     \]

   - Let \(F\) and \(G\) be the projections of \(D\) and \(X\) on \(AC\), respectively. We have:
     \[
     A_{ADC} = \frac{DF \cdot AC}{2} \quad \text{and} \quad A_{ACX} = \frac{XG \cdot AC}{2}
     \]

   - Since \(A_{ADC} = A_{ACX}\), it follows that:
     \[
     DF = XG
     \]

   - Therefore, \(DX \parallel AC\), and \(M\) lies on the line through the midpoint \(E\) of \(BD\) parallel to \(AC\).

4. **Case 2**: \(M\) lies on the half-plane of \(AC\) that contains \(B\).

   - The analysis is similar to Case 1. By reflecting \(D\) over \(M\) and following similar steps, we conclude that \(M\) must lie on the line through the midpoint \(E\) of \(BD\) parallel to \(AC\).

Conclusion:
The point \(M\) must lie on the line through the midpoint \(E\) of \(BD\) parallel to \(AC\).

The final answer is \( \boxed{ M } \) lies on the line through the midpoint \(E\) of \(BD\) parallel to \(AC\).