Abstract Lung cancer is the second most common form of cancer-behind prostate cancer in men, and breast cancer in women-accounting for 15% of all new cases. It is also the leading cause of death among both men and women. Secondary lung cancer is also an important problem which arises from primary tumors in other tissues aggressively metastasizing to the lungs. The lung is a key site of tumor metastasis, and these secondary tumors are very difficult to treat. Treatment of lung cancer is typically invasive and ranges from chemotherapy, to radiotherapy, to surgical removal of the cancerous tissue. However, a new horizon of cancer therapy involves immunotherapy--altering and/or boosting the immune system to help fight cancer. One immune-targeted cancer therapy involves blocking the PD1/PDL1 pathway; a pathway which acts to suppress the immune system by inhibiting T cell signaling. However, little is known about how exactly this and other immunotherapies modulate anti-tumor T cell responses in vivo. We used confocal and multiphoton microscopy to track T cell motility and T cell/tumor interactions in vivo within lung tumors in PDL-1 treated mice. Blockade of PDL-1 altered the spatio-temporal patterning of T cell motility and their tumor killing power, and inhibited lung tumor progression. The knowledge of how αPDL-1 cancer immunotherapy affects the immune microenvironment of tumors can lead to the development of safer, more potent treatments for lung cancer and other forms of cancer.