This document reviews the use of sensors and IoT technologies to optimize agricultural crop yields through precision farming. It discusses how precision farming aims to provide crops with optimal resources like water and fertilizer. Several studies that used sensors to monitor soil parameters like moisture, temperature and humidity are summarized. Wireless sensor networks are shown to effectively monitor greenhouse conditions. System architectures are proposed that collect sensor data and use it to control actuators and make recommendations to farmers. The conclusion is that precision farming enabled by IoT can help manage field variations, grow more food efficiently and reduce costs through accurate monitoring and control of key growing parameters.