Colorectal cancer (CRC) is the second most commonly diagnosed cancer and the third leading cause of cancer-related death in Korea [1]. Because of rapid spread of colonoscopy screening, there was a general decrease in CRC until 2010. However, recent studies have reported that the decline in CRC incidence has reversed during the last few years, especially in middle-aged persons, and the occurrence of early-onset CRC has rapidly increased [2]. Most patients with CRC are diagnosed at an operable stage; however, approximately 20% of patients with stage III or high-risk stage II CRC relapse within 5 years after curative resection [3]. Moreover, the 5-year relative survival rate for metastatic CRC is 14% [2]. To improve clinical outcomes for patients with CRC, a more effective treatment modality is required to fulfill those unmet needs. Cancer is fundamentally a genetic disease since the accumulation of mutations, fusions, and copy number alterations drives tumorigenesis. However, recent research on the tumor immune microenvironment (TIME) has revealed the importance of interactions between tumor cells and surrounding immune cells in tumorigenesis [4]. Immune checkpoint inhibitor (ICI) treatment , such as anti-cytotoxic T-lymphocyte antigen 4 (CTLA-4) antibodies and anti-programmed death-1 (PD-1) antibodies, has shown marked clinical benefits in many types of cancer [5,6]. CRC also holds promise for cancer immunotherapy use, and the U.S. Food and Drug Administration (U.S. FDA) approved immunotherapeutic agents for microsatellite instability-high (MSI-H) CRC in 2017 (Fig. 1) [7,8]. In this review, we describe the landscape of the immune microenvironment in CRC and summarize the clinical usefulness of several suggested biomarkers in CRC immunotherapy. IMMUNE LANDSCAPE OF COLORECTAL CANCERS Historical use of microscopic evaluation of immune environment in CRCs To our knowledge, Spratt and Spjut [9] published the first study on integrative histologic evaluation of CRCs in 1967. In that study, the authors evaluated histologic grade, mucinous elements, depth of invasion, characteristics of tumor border, lym