In order to contain rising healthcare expenditure, the Dutch minister of Health, Welfare, and Sport has agreed with various stakeholders to shift care as much as possible from the expensive specialised care sector to the cheaper primary care sector. Such a shift is only desirable if it constitutes a permanent shift to the primary care sector thereby reducing costs while simultaneously increasing quality of care and the health of the population. In order to prove the success of substitution initiatives, their effects must be compared with the national trend at population level. Longitudinal research is necessary as substitution constitutes a transitional process that requires continuous monitoring and interaction between patients, healthcare providers, insurers, and scientists.