For the first time in human history, we fill – and in many ways surpass – the planet’s natural boundaries. This means that we have a greater responsibility than ever before: for the living beings that share the planet with us, but also for ourselves, by protecting the planetary boundaries that warrant our continued safe existence. Unfortunately, our economic model is poorly adapted, as it is optimized for ever-greater production and consumption of goods in a supposedly unlimited ecosystem. Its proponents hypothesize that a Circular Economy could allow us to decouple wealth and well-being from resource and energy consumption. However, notwithstanding their theoretical potentials, Circular Economy measures have not been able to offset the growth of economies and increased consumption. This chapter presents nine fundamental physical and natural, economical, and logical, and social and organizational boundaries. They include thermodynamics, time, nature, dissipating data, and transaction costs, value add and value loss, demand focus, incentives, solutions, institutional capacity, and minds. While describing limits to what the Circular Economy can achieve, we argue that these can be partially mitigated through systemic, farsighted, and determined policy and business action to accelerate system change.