This book delves into fundamentals of municipal solid waste incinerator (MSWI) ash as it pertains to management and resource recovery. MSWI is a popular waste management strategy globally, and characteristics, regulations, and recycling techniques of the ash byproduct influence the management and upcycling. Due to our modern waste stream, MSWI ash contains trace chemicals that limit use and may require special management and a thorough understanding. The ash also contains components of value (copper, gold) that can be recovered and repurposed, and remaining particles serve as sustainable construction materials. Implementation of MSWI ash resource recovery operations is challenged by heterogeneity, unwanted constituents, and reluctance due to its origin. Here we synthesize current knowledge of global MSWI ash characterization, regulation, beneficiation, and recycling to provide useful resources for stakeholders and decision makers charged with managing this material.