The factors and plant processes that have been proposed to control treeline location are evaluated for European and North American treelines. While often presented as alternative determinants of treeline, the ideas of the increased environmental hazard and the idea of tree material limitation are considered to be complementary. This is because plant damage, rather than outright death, coupled with difficult conditions for material acquisition and small available material pools, eventually makes accretion of biomass untenable. The location where this balance is achieved varies with climate, but catastrophic events and human activities make correlation with climate tenuous and future predictions difficult. Field manipulation of conditions for many tree species in many geographic locations at and below the treeline are needed to develop generalizations that are both explanatory and predictive.