Since GPS is not available inside buildings, indoor navigation systems have to use different techniques. Some of them, such as Wi-Fi and smartphone sensors, are already existent in most situations. Some others, for example beacons or locator nodes (used for analyzing motion profiles) have to be added.
The advantage of Wi-Fi is, that access points are already existent in most cases. One disadvantage is that on Apple devices positioning via Wi-Fi is not possible. Beacons are easily installed and work very accurate (up to 50 cm). Results can be refined by smartphone sensors. Those techniques have one benefit: they can detect the floor level, unlike GPS.