Cycling as a mode of transportation has many advantages for both cyclists and society: it is a low-cost, low-polluting, health-improving way to travel. In light of these benefits, a growing number of cities throughout the world are implementing policies to promote cycling, though they continue to struggle to identify the most effective ways to spend their limited resources. In response, transport researchers are also increasingly focusing their efforts on cycling, addressing a broader range of questions than in the past, including: how much cycling is there? What strategies will increase cycling? What are the benefits of increased cycling? This paper reviews the challenges that researchers face in providing answers to these questions by highlighting limitations of the existing research, identifying remaining research needs, and discussing methodological considerations for addressing those needs. Investments in cycling research can pay off by providing a basis for identifying the most effective strategies for increasing cycling and providing more accurate estimates of the benefits that would accrue.