The successes with immune checkpoint blockade (ICB) and chimeric antigen receptor(CAR)-T-cell therapy in treating multiple cancer types have established immunotherapy as a pow-erful curative option for patients with advanced cancers.Unfortunately,many patients do not derive benefit or long-term responses,highlighting a pressing need to perform complete investiga-tion of the underlying mechanisms and the immunotherapy-induced tumor regression or rejection.In recent years,a large number of single-cell technologies have leveraged advances in characterizing immune system,profiling tumor microenvironment,and identifying cellular heterogeneity,which establish the foundations for lifting the veil on the comprehensive crosstalk between cancer and immune system during immunotherapies.In this review,we introduce the applications of the most widely used single-cell technologies in furthering our understanding of immunotherapies in terms of underlying mechanisms and their association with therapeutic outcomes.We also discuss how single-cell analyses help to deliver new insights into biomarker discovery to predict patient response rate,monitor acquired resistance,and support prophylactic strategy development for toxicity man-agement.Finally,we provide an overview of applying cutting-edge single-cell spatial-omics to point out the heterogeneity of tumor-immune interactions at higher level that can ultimately guide to the rational design of next-generation immunotherapies.