While the voluntary carbon credit market has reached a considerable size, it remains largely unregulated, with significant risks of trading uncertainty and greenwashing. Even the notion of carbon credit and its legal nature are not clear. Nevertheless, investments in carbon credit and companies declaring themselves «carbon neutral» by purchasing carbon offsets are growing fast. In this context, the paper aims to define the phenomenon and to identify its main legal issues in terms of trading certainty and integrity. This is done by considering similarities and differences with regulated markets for emission allowances, such as the EU Emissions Trading Scheme.