This chapter elaborates on the urban ecology approach and its relevance for urban sustainability transitions. It argues that understanding urban areas as complex social-ecological-technical systems (SETS), in which people, ecological processes and built environments are inextricably linked, is pivotal in addressing urban problems precipitated by climate change, inequity, environmental pollution and many other causes. The chapter explicates the urban ecology approach, its value for urban sustainability transitions and the ways in which the approach is pushed by transitions thinking, using a case study of urban vacant lots in NYC. Vacant lots in urban areas can provide important social and ecological benefits to both human and nonhuman inhabitants of the city through the ecosystem services they provide and therefore open up opportunity for sustainability transitions. This case study illustrates the benefits associated with taking an urban ecology approach in making sense of the city.