The last decade has seen a dramatic shift in the focus of nitrogen cycling research in forest ecosystems. Concerns over nitrogen deficiencies and effects of removal in harvest have given way to concerns over excess nitrogen availability and the potential for forest decline and surface water pollution. Driving this paradigm shift is the increase in atmospheric deposition of nitrogen to forests due to industrial and agricultural activity. At the core of the new paradigm is the concept of 'nitrogen saturation' of forest ecosystems. The purpose of this review is to synthesize recent advances in research relating to nitrogen deposition effects on temperate zone forest ecosystems, and the further effects of nitrogen saturation on environmental quality.