Cuprate superconductors are in the material family that have the highest superconducting transition temperature (T c ) of ~130 K under ambient pressure. The infinite-layer structure is the essential building block of high-T c cuprates with a T c of over 100 K and is therefore vital to understand the mechanism of high-T c superconductivity. While the infinite-layer phase is inaccessible using bulk single-crystal synthesis methods, we synthesized single-crystalline thin films of the infinite-layer cuprates using our unique oxide molecular beam epitaxy setup. To clarify the relationship between their microscopic crystal structures and electronic responses, we performed atomic-resolution electron microscopy measurements of the CuO 2 (copper peroxide) planes in the infinite-layer cuprates, the playground of high-T c superconductivity.