Two separate, but interacting, global agendas are now leading to new, additional requirements for the future development of cities: The UN Agenda 2030, putting cities at the heart of sustainable urban development with its Sustainable Development Goal (SDG) 11, and the Paris Agreement COP21 adopting the 1.5 °C target as a basis for global emissions reductions. Regulators and researchers have widely recognized the necessity to put cities, as an important object of assessment, and city authorities, as an important actor group, at the core of climate mitigation efforts. For cities themselves this topic becomes a factor of competition among peers. In their pursuit of a low carbon future, however, they are confronted with a number of theoretical and practical questions regarding target setting and subsequent planning for mitigation. As a contribution to the current discussion, the paper initially clarifies on which principles the allocation and accounting of city-related greenhouse gas (GHG) emissions are typically based. A good understanding of the GHG sources and reduction potentials is essential for defining feasible targets and designing efficacious reduction strategies. ... mehrBuilt on this, the paper then presents how climate targets are defined at city level and analyses the methodological considerations that arise in the case of target-setting approaches involving bringing the emissions balance to zero. Although first definitions of "net zero emission" concepts on an urban scale can be found in literature, their precise meaning and applicability still remain vague, with unclear system boundaries, calculation and assessment rules. This paper provides a definition framework for clarifying such concepts.