This multi-case study analyzed the formal and informal development of adaptive capacity in fire service leaders within the context of increasingly complex situations driven by urbanization and climate change. Triangulating interviews, observations, and documentation review, this study included two urban fire service cases impacted by past complex climate events: City of Gatlinburg Fire Department in Tennessee where Chimney Tops II mega fire occurred in November 2016 and the fire service in Harris County, Texas where extreme flooding from Hurricane Harvey occurred in August 2017. This study sought to answer the primary research question: "How do fire service leaders develop adaptive capacity within complex situations?" Measured in lives and structures lost, the problem of increasing complexity in the fire service increases the criticality of localized adaptivity. This study contributes new empirical insight into how organizations support and enable leaders' development of adaptive capacity. This study synthesized aspects of professional responsibility and complexity leadership theories to conceptualize the complex and socio-material (webs of connected human, social, and material) dynamics that give rise to and support leader development of adaptive capacity. This study found three interconnected practices of shift duty, individual professional development, and incident response within and across which leader adaptive capacity developed. Moments of emergent leadership and professional learning found in this study developed leader adaptive capacity across these practices. These developmental moments broadened the socio-material webs which leaders leveraged to adapt within the complex situations of incident response. This study's practical implications may benefit several audiences including leaders in the fire service and other high stakes professions, those that design and facilitate leader development, and professional associations, among others. This study also contributes one theoretical lens to delve into the complex and socio-material aspects of leader development of adaptive capacity. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]