Climate is a critical factor affecting forest ecosystem and their capacity to produce goods and services. This chapter reviews published studies of climate-forest relationships with emphasis on indications and mechanisms of change during recent decades. Effects of climate change on forests depend on ecosystem-specific factors including human activities, natural processes, and several dimensions of climate (temperature, drought, wind, etc.). Indications of recent climate-related changes in ecosystem processes are stronger in boreal forests than in other domains. In contrast, constraints on adaptive capacity that increase vulnerability to climate change are generally more severe in subtropical and tropical forests than in temperate and boreal domains. Available information is not sufficient to support a quantitative assessment of the ecological, social and economic consequences of recent forest responses to human influences on climate. The complexity of natural and human systems is a formidable barrier to impact quantification and predictability. For example, effects of land use practices and invasive species can overshadow and interact with effects of climate change. Nevertheless, substantial progress has been made in defining mechanisms of climate-change impacts on forest ecosystems. Knowledge of impact mechanisms enables identification and mitigation of some of the conditions that increase vulnerability to climate change in the forest sector.