 Natural fibers are increasingly being used as reinforcements in polymer composites due to their abundance, low-cost, lightweight, renewability, biodegradability, and high-specific properties. This paper reviews the different sources of natural fibers, their properties, modifications, and effects of treatments on natural fibers. It also summarizes the major applications of natural fibers and their effective use as reinforcement for polymer composite materials. This article was authored by Yashas Ghoda Thayavihalli Jirihappa, Sanjay Mavinkara Rangappa, Jyotishkumar Parameswaran Pillai, and others.