The first step of QSD is to define the problem space. In essence, this is the process of specifying the technology of interest, what will be included or excluded from the analysis (i.e., the system boundary), and the ranges of assumptions that can be made for QSD inputs. QSD inputs comprise any assumptions that may influence the performance and sustainability of the system, including decisions about the system design or operation of the system (i.e., decision variables), characteristics of the technology or its components (i.e., technological parameters), and the context in which the technology can be deployed (i.e., contextual parameters). The QSD inputs form an N-dimensional "problem space" encompassing all possible combinations of values of the QSD inputs (N is the number of QSD inputs), and the sustainability of the system will be evaluated within this space through QSD. To help familiarize the reader with these terms and their meaning, a summary of terminology definitions can be found in Table and specific examples from the literature can be found in Table and Table (Electronic Supplementary Information, ESI).