This article presents an overview of battery-free wireless sensing nodes primarily for IoT applications that are typically deployed in a large sensor network. Powering strategies focus primarily on ambient energy harvesting and far-field wireless power transfer (WPT) techniques, and considerations encompass the entire sensor network of potentially hundreds of sensors. Various approaches are compared to significantly reduce power consumption including wake-up receivers (WURs) designed with far less power consumption than the main transceiver, and making use of backscattering for data transmission. A comprehensive comparison of sensor node characteristics from recent research highlights the constant push forward in this field.