Planners and policy makers responsible for various economic establishments such as cities and other resource conservation districts have an unrecognized ethical responsibility to their constituents: the pursuit of perpetual sustainability. Perpetual sustainability involves, as much as is practicable, infinite time horizons on the scientific, economic, and social components of critical infrastructure decision making. Failure to resolve long-term sustainability threats can lead to avoidable disasters that destroy lives and fortunes. In the early months of 2018 Cape Town, South Africa, a city of over four million inhabitants, narrowly escaped shutting down the municipal water supply system after its reservoirs fell below 6.5% useable capacity before “Day Zero” was pushed back into 2019. On the brink of disaster, Cape Town has taken amazing steps to reduce consumption and buy time but the effort and investment come too late to mitigate a disaster that experts have identified as years-in-the-making. Another 11 major cities around the world are presently under severe water stress but, like Cape Town, local leaders have focused on engineering solutions to the water crisis: acquiring additional supply or consuming the present supply more efficiently. These approaches can be successful but planners must recognize and respect the natural resource limits of their local area and recognize sustainability-based limits-to-growth far in advance of the taps running dry. The authors argue, with regard to sustainability, that “in perpetuity” is the minimum acceptable measure of duration and that perpetual sustainability is no less than an ethical mandate for natural resources and critical infrastructure decision making.