Irrigation is now recognized as an important component in the agriculture economy of Mediterranean regions. As practiced by many growers, it is often based on traditional application methods that fail to measure the supply of water needed to satisfy the variable requirements of different crops. In order to achieve more profitable and sustainable cropping systems, it is essential to modernize existing irrigation systems and improve irrigation water use efficiency (WUE). Up-to-date methods of irrigation should likewise be based on sound principles and techniques for attaining greater control over the soil-cropwater regime and for optimizing irrigation in relation to all other essential agricultural inputs and operations. Accurate predictions of crop water requirements are necessary for an efficient use of irrigation water in container crops cultivated both outdoors and in greenhouse. Irrigation scheduling (IS) has conventionally aimed to achieve an optimum water supply for productivity, with soil or container water content being maintained close to field capacity. Different approaches to IS have been developed, each having both advantages and disadvantages but despite the number of available systems and apparatus, not entirely satisfactory solutions have been found to rationalize IS, assuring optimal plant growth with minimal water use (Jones, 2004). Many growers, especially in the Mediterranean regions, use simple timers for automated irrigation control of containerized crops and scheduling is based on their own experience. According to Baille (2001), the available advanced methods for controlling irrigation at the short-term decision level are based either on climate or on soil moisture status. In the climate-based method, crop water use is computed by means of algorithms that estimate ET using meteorological data. The Penman-Monteith (PM) equation (Monteith, 1973; Stanghellini, 1987) and its simplified versions (e.g. Baille et al., 1994) have been used for predicting ET in many container-grown crops, where substrate evaporation losses are generally negligible and ET is determined almost exclusively by crop transpiration. The soil-based method uses the measurement of soil water potential or content. A combination of climate and soil-based methods would be recommended, because this allows a check of the coherence and concordance between data regarding soil moisture and crop water demand thus making IS more reliable and accurate. In this chapter, different approaches for ET modelling in container crops grown both in greenhouse and outdoor will be described and its application to IS is briefly discussed.