The implementation of near-field communication (NFC) technology in smartphones has grown rapidly, especially due to the use of this technology as a payment system. In addition, the ability to use the energy transmitted not only for communication, but also for feeding other devices, which together with the low cost of NFC chips and the internet connectivity of the smartphones, allows the design of battery-less RF tags with sensing capabilities, whose information can be sent to the cloud. This is of great interest in the increasing amount of IoT (Internet of Things) scenarios. This book studies the feasibility of these sensors, analyzing the different parameters that have an influence on performance and in the range of operation. It also presents techniques to increase the range and analyzes the effects of certain materials when they are close to the antenna. The design and analysis of several sensors that can be powered and read by any NFC enabled device are presented in this work.