Root hairs are found on most terrestrial flowering plant species. They form from epidermal cells at a predetermined distance behind the growing root tip in three main patterns. Their presence, pattern, length, density and function are genetically controlled and numerous genes are expressed solely in root hairs. Their growth and proliferation are attenuated by the environment and root hairs growing in soil are generally shorter and less dense than those in laboratory studies. Root hairs have a number of functions including anchorage, root soil contact and bracing to enable roots to penetrate hard soils. However, their primary function is acquisition of nutrients and water, in particular phosphate. They are the site of transporters, exudation of active compounds and infection point of symbiotic microbial interactions. They have a profound effect on rhizosphere characteristics and are a potentially useful target for breeding crops for future agricultural sustainability.