This paper explores the design considerations and hurdles encountered by the CHinA National CancEr Screening (CHANCES) Trial and the Tomosynthesis Mammographic Imaging Screening Trial (TMIST), both aimed at advancing cancer screening research. Before population-based cancer screening programs are launched, it is important to have confidence that the potential benefits of the screening process and resulting interventions outweigh harms, an ethical imperative because the people actively invited into the programs are relatively healthy. Large randomized screening trials provide the strongest, direct evidence regarding the balance of benefits and harms. The implementation of cancer screening programs involves a series of steps, with outcomes influenced by factors such as the prevalence of the disease, availability of effective treatment within the health-care system, and acceptance by the target population-all of which may vary considerably from country to country. This paper examines how these factors shaped the design and statistical approach of the CHANCES Trial for lung and colorectal cancers and the TMIST trial for breast cancer. We discuss the rationale, objectives, endpoint definitions, trial designs, and sample size considerations, highlighting both the challenges and opportunities presented in different settings. Ultimately, the goal is to foster collaboration and develop screening strategies that are scientifically robust and practically effective for diverse populations worldwide.