How to spy on someone without them knowing?

To spy on someone without them knowing became possible thanks to new technology – spyware. It is a stress-free way to view what another person is doing on their device and this way get to know everything about them. Why does its use come without stress?
The app runs in a stealth mode and is not visible on the target device. However, for certain spy applications, its icon might be optional. The user is the only one who knows about it. Potential users may find convenient the following spy applications: Highster Mobile, Spyzie, FoneMonitor, Spymyfone, mSpy.