Sporadic colorectal cancer has traditionally been viewed as a malignancy of older individuals. However, as the global prevalence of the disease diagnosed in younger individuals (<50 years) is expected to increase within the next decade, greater recognition is now being given to early-onset colorectal cancer. The cause of the predicted rise in prevalence is largely unknown and probably multifactorial. In this Series paper, we discuss the potential underlying causes of early-onset colorectal cancer, the role of energy balance, biological and genomic mechanisms (including microbiome aspects), and the treatment of early-onset colorectal cancer. We have specifically considered the psychosocial challenges of being diagnosed with colorectal cancer at younger age and the potential financial toxicity that might ensue. This Series paper brings a comprehensive review based on the existing data in the hopes of optimising the overall outcomes for patients with early-onset colorectal cancer.