This chapter examines the eight most common challenges addressed by green growth policy recommendations in OECD Economic Surveys, Environmental Policy Reviews, Investment Policy Reviews and Reviews of Innovation Policy (Figure 2.1).2 The challenges relate to implementing market instruments to price pollution and natural-resource use; orienting tax systems to advance green growth; designing environmentally relevant subsidies; and gearing sectoral policy towards green growth. Green growth policy design and mainstreaming are ongoing, the range of relevant issues is broad and the challenges countries face are different (Table 2.1). The eight most common challenges addressed in this chapter will not necessarily apply to all countries. Rather, they demonstrate where some of the main opportunities to accelerate and improve green growth policy implementation lie across countries. Other challenges remain relevant, depending on the country in question. To transition to a green economy, governments need to send consistent market signals that the cost of carbon dioxide (CO2) and other greenhouse gas emissions will gradually increase - otherwise companies will have little incentive to shift away from fossil fuels.