An auditor is an external evaluator employed to certify the claims that a company makes, typically in relation to its financial position.

With the rise of ecosystem services and carbon trading, the market in environmental auditing has also grown.

In the case of the Clean Development Mechanism, private auditors (referred to as Designated Operational Entities) are employed to assess whether projects should be registered (‘validation’) and whether carbon credits should be issued (‘verification’). Critics contend that this outsourcing of regulation can generate conflicts of interest, and lead to lax standards.