This Practice Note is concerned with challenges to funding arrangements associated with retainers and considers what is the definition of a funding arrangement (ie a retainer-based funding arrangement or a separate contractual funding agreement between the client and a non-legal representative third party funder (also known as a pure funding arrangement)), and why its parties would want to challenge a funding agreement or a funding arrangement. It also covers how to challenge a retainer-based or pure funding arrangement including establishing the terms of the funding arrangement using disclosure and other means. Finally, it considers common challenges to retainers, and two types of funding agreements: conditional fee agreements (CFAs), damages based agreement (DBAs) as well as pure funding arrangements with professional litigation funders (contract based funding agreements), including challenges based on champerty.