BTK (Bruton’s tyrosine kinase) has become a key therapeutic target across several hematologic diseases, beginning with its original use in CLL/SLL. As a central mediator of B-cell receptor signaling and microenvironment interactions, BTK supports survival, proliferation, and trafficking in multiple mature B-cell malignancies (mantle cell lymphoma, marginal zone lymphoma, Waldenström macroglobulinemia, and other indolent/aggressive lymphomas) and in selected immune-mediated conditions such as chronic graft-versus-host disease. Covalent BTK inhibitors (ibrutinib, acalabrutinib, and zanubrutinib) irreversibly bind the C481 residue and have produced high response rates and durable disease control, often replacing chemoimmunotherapy in the relapsed setting and, for some entities, even in the first line. Differences in kinase selectivity lead to different safety profiles: second-generation covalent agents generally maintain efficacy while reducing significant off-target toxicities, especially atrial fibrillation and hypertension. Resistance to covalent BTK inhibitors most commonly develops through BTK C481 substitutions and activating PLCG2 mutations, with other kinase-domain variants increasingly recognized. Non-covalent BTK inhibitors (e.g., pirtobrutinib) bind BTK independently of C481, can overcome classic C481-mediated resistance, and extend BTK pathway targeting into later lines of therapy. Overall, BTK inhibition has evolved into a versatile platform enabling long-term, often chemo-free management strategies.