Provided are methods of increasing nitrogen use efficiency, yield, biomass, growth rate, vigor, oil content, fiber yield, fiber quality and/or abiotic stress tolerance of a plant by expressing within the plant an exogenous polynucleotide comprising a nucleic acid sequence at least 80% identical to SEQ ID NO:1-467, 785-3047; or an exogenous polynucleotide encoding a polypeptide at least 80% identical to SEQ ID NO:468-784, 3048-4333, 4335-4682. Also provided isolated polynucleotide comprising a nucleic acid sequence selected from the group consisting of SEQ ID NOs:1-467, 785-3047, which can be used to increase nitrogen use efficiency, yield, biomass, growth rate, vigor, oil content, fiber yield, fiber quality and/or abiotic stress tolerance of a plant.