Despite the increased attention and capital incentives around corporate sustainability, the development of sustainability reporting standards and monitoring systems has been progressing at a slow pace. As a result, companies have misaligned incentives to deliberately or selectively communicate information not matched with actual environmental impacts or make largely unsubstantiated promises around future ambitions. These incidents are broadly called “greenwashing,” but there is no clear consensus on its definition and taxonomy. We pay particular attention to the threat of greenwashing concerning carbon emission reductions by coining a new term, “carbonwashing.” Since carbon mitigation is the universal goal, the corporate carbon performance data supply chain is relatively more advanced than that of the entire sustainability data landscape. Nonetheless, the threat of carbonwashing persists, even far more severe than general greenwashing due to the financial values attached to corporate carbon performance. This chapter contextualizes sustainable finance-related carbonwashing via an outline of the communication as well as the measurement, reporting, and verification (MRV) of carbon emission mitigation performance.