Abstract The concept of sustainable development is gaining ever more interest in the political discussion. However it is often overlooked that this concept has important repercussions for technological development. This is especially true for process industry as this sector is responsible for most material flows within human society as well as the exchange of material and energy with the environment. Based on an operationalised set of criteria for sustainability and on conventional mass and energy balances, the concept of the sustainable process index (SPI) measures the potential impact (pressure) of processes (or more generally ‘activities') on the ecosphere. The SPI compares mass and energy flows induced by human activities with natural flows [Krotschak C, Narodoslawsky M. The sustainable process index—a new dimension in ecological evaluation. Ecological Engineering 1996;6(4):241]. As natural flows are always linked to area (examples are the growth of biomass, precipitation and, most importantly, solar radiation) the basic unit of the SPI is area. It is the total surface area that is required by any activity that exchanges material with the environment to be “sustainably embedded into the ecosphere (=environment)”. Integrated assessment of processes with the SPI aggregates resources as well as emissions to the three different ecological compartments air, water and soil [Krozer J. Operational indicators for progress towards sustainability (no. EV-5V (T94-0374). EU project final report. Den Haag (The Netherlands), TME, 1996]. The lower the requirement of area for a given activity is, the lesser is the impact of this activity on the environment. The SPI concept allows a quick and reliable evaluation of very diverse processes according to their environmental impact from a sustainable development point of view [Krotscheck C. How to measure sustainability? Comparison of flow based (mass and/or energy) highly aggregated indicators for eco-compatibility. EnvironMetrics, 1997; 8: 661]. It uses data available to a process engineer even at a very early stage in process development and may therefore be used as a tool for optimising processes in the course of their development.