Tyrosine kinase inhibitor (TKI) therapy has revolutionized cancer treatment by selectively blocking specific target oncogenes and signaling pathways crucial for tumor growth, thereby offering improved outcomes for a large number of cancer patients. However, despite their initial effectiveness, resistance to TKIs remains a significant challenge in clinical practice. In this chapter, we summarized various mechanisms contributing to acquired TKI resistance, including tyrosine kinase-dependent and independent mechanisms. The former mechanism includes acquired mutations, amplifications, or fusions of the target genes, activation of alternative receptor tyrosine kinase pathways, and downstream signaling pathways. The latter mechanism includes epigenetic modification, histologic transformation, cancer stem cell, cell metabolism reprogramming, cell death inhibition, tumor microenvironment, drug delivery abnormality, and so on. Understanding the mechanisms underlying TKI resistance is paramount for improving patient outcomes and developing more effective treatment strategies.