Some encompassing terminology is required in order to accommodate different conceptual approaches in the three pillars of sustainability. So, this chapter provides a literature review exploring the threshold concept. In environmental research – especially in ecology – thresholds are often associated with limits which have certain system-inherent processes. In social and economic disciplines, if the notion of limit or critical limit is present, the concept of targets is often more appropriate which are linked to political objectives and social acceptability. The concept of threshold is accommodated within the general framework of limits and targets. What is important is the understanding developed here that almost any environmental, social or economic system has the potential to reach a point or an area that is unsustainable, or outside acceptable limits, relevant at a regional level. When identifying values for limits, a number of issues need to be considered. The consequences of exceedance of limits depend to a large extent on two related concepts, more or less relevant for both environmental and socio-economic sciences: path dependency and eversibility. Together,these help understand what the socio-economic and environmental consequences are, if they are reversible and the likely cost of achieving reversibility, or whether exceedance precludes any recovery. Exceedance of environmental limits often has a direct cost, revealed across many sectors, whereas the costs associated with exceedance of socio-economic limits may be harder to quantify. Together with a concept of risk, these concepts lead us to apply the precautionary principle, in other words to set conservative limits that define ‘unacceptable consequences’ some distance in advance of the point (or area) at which system break down or severe damage occurs. Crucially, these limits are derived through deliberative processes and involve both social acceptability and political input, together with scientific understanding of how the system operates (be it socio-economic or environmental). Firstly, the paper explores the concepts of targets and limits from environmental and socio-economic perspectives and suggests some unifying terminology. Secondly, we examine some of the issues of uncertainty in considering values for limits or targets. These issues deal with the notion of equilibrium, the understanding of complex processes and the capacity of a system to adapt to an external event. Thirdly we underline how this uncertainty in the regional assessment challenges our ability to predict the consequences of exceeding the limits.