California is facing a warmer climate over the next century. Evidence already exists that severe heat events pose considerable health risks, and new evidence shows that the character of those heat events is changing. Heat events are becoming progressively more humid, lasting longer than average, and occurring in areas not as accustomed to extreme-heat. From a public health perspective, the lack of clear heat wave definition can cause confusion when planning around new heat extremes. In the face of a changing climate, this paper finds that definitions based on aspects of human health may offer a more accurate basis for planning and preparedness. Planners and practitioners in the fields of public health and urban design will increasingly need to incorporate changing patterns of extreme-heat into long-term planning. Our aim is to equip them with a baseline from which to judge the influence of climate change on heat vulnerability in their local area. We find less-stringent, health-informed heat wave thresholds may better represent heat sensitive populations. Utilizing a simple statistical framework, we generate over 63 unique, health-informed heat thresholds tailored to CaliforniaÕs diverse tapestry of climates and demographics. Using these thresholds as a baseline, we then generate probabilistic climate projections to evaluate how the signatures (e.g., severity, frequency, duration, and timing) of heat-health events are changing. Census tract-level vulnerability maps are also provided to help identify existing areas of need. We conclude that long-term heat adaptation efforts are particularly urgent in coastal, agricultural, and increasingly urbanized regions of California where sensitive populations will face, on average, longer and increasingly severe heat-health events. This document provides a description of the methods, findings, and limitations behind the building of an online interactive tool, the California Heat Assessment Tool (CHAT) (http://www.cal-heat.org). The aim of the tool is to support the inclusion of extreme-heat considerations into long-term policy and planning decisions throughout California. Given the multi-faceted nature of heat vulnerability, we also hope that this tool will empower local practitioners to better communicate the urgency of this issue to build much needed support for improved planning and adaptation solutions.