Abstract: Tumor-infiltrating lymphocytes (TILs), an important part of the tumor microenvironment (TME), play a crucial role in the occurrence and development of malignant tumors. TILs contain cell subpopulations that have both pro-tumor and anti-tumor effects. TIL-based immunotherapy has achieved good results in most solid tumors. TIL numbers and TIL subgroup composition in tumors are of great value for judging patient prognosis. This review discusses TIL composition, bidirectional effects, mechanisms of action, and progress in clinical research focused on TILs in common solid tumors.