Provided are isolated polynucleotides comprising a nucleic acid sequence at least 80% identical to SEQ ID NO:619, 617, 606, 615, 629, 1-36, 40, 41, 43-45, 49, 52-56, 58, 113-343, 351, 354-358, 605, 607-614, 616, 618, 620-628, 630-638, 642, 645, 650, 651, 670, or 671. Also provided are nucleic acid constructs comprising same, isolated polypeptides encoded thereby, transgenic cells and transgenic plants comprising same and methods of using same for increasing abiotic stress tolerance, yield, biomass, growth rate, vigor, oil content, fiber yield, fiber quality, and/or nitrogen use efficiency of a plant.