This report outlines the Dutch model of Horizontal Monitoring (HM), which is widely regarded as one of the first examples of a cooperative compliance program. It describes how, since 2005, the Netherlands Tax and Customs Administration (NTCA) developed a monitoring regime that has significantly altered the relationship between the Dutch tax authority and corporate taxpayers. The report demonstrates that under HM the attitude of both corporates and tax administrators has shifted from an adversarial ‘them and us’ relationship, to one stronger characterised by cooperation. Despite the widely identified benefits of HM, including increased ability of corporates to acquire fiscal certainty, the monitoring regime faces major challenges. It has proven particularly difficult to quantify the model’s impact on revenue collection and the tax authority’s administrative resources. The report concludes that if HM is to subsist, it is vital to increase formalisation and transparency of the risk monitoring techniques as applied by the tax authority, and develop more advanced metrics than have been available hitherto.