Single cell transcriptome analysis became an essential tool to define cell types or sub-populations within a heterogeneous bulk population. Tumor-associated microenvironment is a complex ecosystem with numerous cell types, supporting the tumor growth, angiogenesis, immune evasion, and metastasis. With the success of checkpoint inhibitors targeting the immune cell compartment, tumor microenvironment is an emerging anti-cancer target and its understanding becomes an imminent issue in the cancer biology.