Put simply, verification is the practice of confirming whether an assertion is true, in that it conforms to set expectations under an agreed level of scrutiny. Verification plays a central role in substantiating the validity of credits in environmental markets. Verification of credit-generating projects includes administrative and technical review, as well as confirmation that the project has been implemented as promised. Verification systems are used to support programs in the regulatory and voluntary crediting context, and to support robust internal tracking systems. Verification systems should seek to provide trusted confirmation that credits represent real environmental benefit. Those designing a verification system will need to make decisions about who conducts verification review and what qualifications they need, what information is reviewed, and the frequency with which verification should occur. Options and examples are used to explore tradeoffs inherent in making these decisions such that the resulting system supports transparency and accountability, ensures costs do not detract from the ability to provide greater environmental benefits, and builds opportunities to learn and improve programs quickly.