The sustainability assessment approach utilized in the development of the WA-PA-SU project sustainability rating system includes three distinctive areas of knowledge: sustainability, continuous performance improvement, and multi-criteria decision-making. Previously, the study of sustainability led to (1) concluding the need for the development of a rating system for industrial projects, with a particular application to oil sands and heavy oil projects; (2) defining the structure of the rating system; and (3) assisting in the pre-selection of sustainable development indicators (SDIs) for surface mining operations. Assessing the sustainability of projects at certain points in time required the application of a methodology selected by the interested groups and/or stakeholders; however, measuring the improvement of projects in sustainable development performance over time (i.e., continuous performance improvement) presents additional challenges. Certain industries (i.e., oil & gas), projects (i.e., oil sands or heavy oil), or specific operations (i.e., surface mining) require a rating system with a particular level of flexibility, offering the opportunity for developers to improve the performance of operations, and for stakeholders to understand the difficulties-and benefits-of implementing SDIs and perform up to levels of truly sustainable development. The present manuscript introduces the performance improvement factor (PIF), which can be determined using three different methodologies: relevance factor or subjective stakeholder valuation, comparative assessment methods, and links to metrics. Additionally, the continuous performance improvement (CPI) indicator measurement is suggested and discussed for a pre-selected set of SDIs for surface mining operations in oil sands projects. Finally, a brief preamble discusses the proposed integrated approach for sustainability assessment and the part it plays in continual performance improvement, offering a foreword to upcoming manuscripts that discuss the other complementary parts of the integrated approach.