The techniques of wireless power transfer has been gaining increasing popularity in the recent years due to the widespread use of mobile electronic devices such as laptops, mobile phones, wearable electronics and wireless sensors. Traditionally, these devices are powered using a battery that needs to be recharged. Wireless power transfer has the potential of enabling new applications of those and many different devices, where it is not possible to use the battery (e.g. due to its size or weight), or where recharging the battery using a wired charger is not practical (e.g. wireless sensor networks). In this thesis we investigate Wireless Power Transfer Networks (WPTNs). These are the networks of wireless power transmitters and receivers that are connected together in a network with the goal of increasing network efficiency through maximizing charging of receivers while minimizing the consumption of energy in the network. In this thesis, the architecture of devices in such WPTNs has been proposed. Subsequently, a physical prototype of such networks has been designed and build. Finally, two fundamentally different approaches to achieving high efficiency has been proposed and analyzed in this thesis. After designing and creating the proof-of-concept WPTN system, its performance has been analyzed, modeled mathematically and verified experimentally. A number of performance measures of WPTNs has been proposed and two approaches to solving the optimization goal of WPTNs have been compared using these measures. Finally, certain conclusions have been drawn on the trade-offs between the first --- simpler approach, and the second --- more complex approach.