This review is concerned primarily with the nature and origin of the forces bringing about movement of water from soil or other media surrounding roots into the conducting elements of the xylem, and with those internal and external factors which affect the rate of such movement. Absorption of water is not an independent process, but is closely related to other processes included in the domain of plant water relations. The rate of water intake is markedly affected by the rate of transpiration and by the extent and condition of the root systems. It is also affected by such environmental factors as the available moisture content of the soil, soil temperature, soil aeration, and to a lesser extent by the kind and concentration of ions in the soil. Some phases of the work discussed in this review naturally are incomplete and do not permit formulation of definite conclusions concerning certain aspects of the absorption problem. Where the writer has drawn conclusions or made generalizations they seem to be those most justifiable on the basis of the available evidence. It is realized, however, that as knowledge of these processes increases it may be necessary to modify some of the present conclusions. It is impossible to cite all the literature, but most of the papers cited have bibliographies in which the reader can locate other papers dealing with any particular phase of the field.