In Unity development, understanding mouse input is crucial for interacting with both 2D UI elements and 3D game objects, necessitating a conversion from 2D screen coordinates to 3D world space positions. The article explores three methods of detecting mouse input in Unity, emphasizing the transition from the legacy input module to the new input system, which offers a modern approach to configuring, reading, and debugging inputs. The legacy module allows for basic mouse position and button state detection using pixel coordinates, but the new input system provides enhanced capabilities through input actions, enabling the integration of multiple input devices and offering better APIs and debugging tools. While the legacy input module remains suitable for simple projects targeting limited platforms, the new input system is recommended for more complex projects requiring diverse input device support. The article also highlights the importance of choosing the appropriate input system early in the development process to avoid costly changes later.