The current main goal of diagnostic medicine is to detect crucial events in 'infinitely' small samples. The key question now is how to determine whether the rare cell events isolated and characterized from these samples reliably reflect the disease and heterogeneity of the tumor. In this review, we provide a short overview of the most recent methods developed for the isolation and characterization of rare cell events in clinical practice, with a specific focus on circulating tumor cells. We discuss the biological value to studying these cells at the single cell level and how these rare cell events can reflect tumor heterogeneity. The potential biomedical applications are also critically discussed in light of precision medicine.