Intratumoral heterogeneity has been identified as one of the strongest drivers of treatment resistance and tumor recurrence. Therefore, investigating the complex clonal architecture of tumors over time has become a major challenge in cancer research. We developed a new fluorescent "optical barcoding" technique that allows fast tracking, identification, and quantification of live cell clones in vitro and in vivo using flow cytometry (FC). We optically barcoded two cell lines derived from malignant glioma, an exemplary heterogeneous brain tumor. In agreement with mathematical combinatorics, we demonstrate that up to 41 clones can unambiguously be marked using six fluorescent proteins and a maximum of three colors per clone. We show that optical barcoding facilitates sensitive, precise, rapid, and inexpensive analysis of clonal composition kinetics of heterogeneous cell populations by FC. We further assessed the quantitative contribution of multiple clones to glioblastoma growth in vivo and we highlight the potential to recover individual viable cell clones by fluorescence-activated cell sorting. In summary, we demonstrate that optical barcoding is a powerful technique for clonal cell tracking in vitro and in vivo, rendering this approach a potent tool for studying the heterogeneity of complex tissues, in particular, cancer.