Abiotic stress factors, such as drought, high salinity, extreme temperatures and chemical pollution, are a serious threat to agriculture and lead to deterioration of the environmental conditions. Drought, which is connected to the greenhouse effect and caused by the global atmosphere pollution, can lead to high soil salinity worldwide. Drought is one of the most unfavourable environmental stress factors for the plants. The clarification of the physiological, biochemical, biophysical and molecular responses of the plants subjected to drought stress is of major importance for the understanding of the mechanisms plants use to increase their resistance to water deficit. Plants under drought stress employ a wide range of responses, from changes of the gene expression and cell metabolism to changes in both growth and yield rates. This is a review of the current knowledge of plant responses to water deficiency from molecular to ecological levels.