Low phosphorus (P) availability is considered a major constraint for plant growth and crop productivity; therefore, the ability of plant roots to acquire P from soil and the mechanisms that regulate phosphorous homeostasis in the plant are topics of great interest. Low P availability elicits a Pi-starvation response that includes morphological, metabolic, and physiological changes oriented to increase P availability and the efficiency of P uptake and usage by the plant. Recent advances in the study of the plant responses to low P availability allowed the identification of several key molecular components of the P rescue system. However, the complete signaling pathways, as well as the putative phosphate receptors, remain largely unknown. In this chapter, we review current research aimed at dissecting the components of the biochemical, molecular, and physiological adaptations associated with the plant responses to P starvation and its relation to the efficiency and effectiveness of P uptake and assimilation from rhizosphere.