How to spy on someone’s cell phone without them knowing?

To spy on someone’s cell phone without them knowing is practicable with spyware. When a user installs it on the target device and makes it work, its owner has no idea it happened. Why? Spyware or spy application runs in the background and its icon is invisible on the phone’s home page. One can find effective the following spy apps: Copy9, Spyzie, TheTruthSpy, Spy Phone App, mSpy.