1. **State the problem:** We need to find the measure of each interior angle in a regular quadrilateral. 2. **Formula for interior angles of a regular polygon:** The measure of each interior angle in a regular polygon with $n$ sides is given by $$\text{Each interior angle} = \frac{(n-2) \times 180^\circ}{n}$$ 3. **Apply the formula:** For a quadrilateral, $n=4$. $$\text{Each interior angle} = \frac{(4-2) \times 180^\circ}{4} = \frac{2 \times 180^\circ}{4}$$ 4. **Simplify the expression:** $$= \frac{360^\circ}{4}$$ $$= 90^\circ$$ 5. **Interpretation:** Each interior angle of a regular quadrilateral (which is a square or diamond-shaped square) measures $90^\circ$. **Final answer:** Each interior angle is $90^\circ$.