Trickle irrigation, sometimes known as drip or micro-irrigation, typically involves dripping water into the soil at very low rates (2-20 L/h) using outlets called emitters (Ross et al. 1978; Jensen 1980; Lamm et al. 2006; Mane 2006). These are placed close to plants to ensure targeted application to the root zone. This chapter focuses on surface trickle irrigation. Other types of trickle irrigation system are bubbler systems, which discharge small continuous streams of water, and spray irrigation systems, which use micro-sprinklers to spray water near the root zone (Schneider and Howell 1999). The latter are generally used on tree and vine crops.