Abstract: A main motivation for the implementation and change of urban rules was the attempt to address urgent environmental challenges. The rules to secure sufficient light and air were geared towards the improvement of environmental conditions, and had– sometimes unanticipated – impacts on the transformation of cities' street spaces. Hong Kong and Macau are two of the densest cities in the world. The implementation, change and repeal of urban rules played a key role in the formation and transformation of their urban forms and spaces. This paper explores how both cities dealt in the past and today with environmental challenges, and particularly those that are related to high density and the change of street spaces. How do they succeed or fail to balance economic interests and concerns for a better environment, health and wellbeing? How do the governments provide development intensity concessions to encourage green features? Currently, Macau for instance is considering the repeal of its "shadow rule", which had been repealed in Hong Kong in the 1980s imposing a fundamental impact on its urban form and street spaces. The paper compares which urban rules were implemented to make both cities greener and its inhabitants healthier while searching for the impact of these rules in the cities' streets. The paper also highlights what complications certain rules caused as a result of "greening incentives" in such high-density cities.