This paper explores the role of low-lying meson resonances (spin ≤ 1) in the context of chiral perturbation theory (CHPT) for pseudoscalar mesons. The authors determine the contributions of these resonances to the coupling constants of the O(p^4) effective chiral Lagrangian, which describes strong interactions at low energies. They find that these resonance contributions dominate the low-energy coupling constants, particularly for vector and axial-vector mesons. The analysis supports the vector meson dominance hypothesis, suggesting that vector mesons play a prominent role in low-energy hadronic interactions. Additionally, the authors calculate the electromagnetic pion mass difference using explicit resonance fields, demonstrating that the low-energy constant at O(e^2 p^0) is also dominated by resonance contributions. The paper concludes with a systematic treatment of all low-lying meson resonances in CHPT, emphasizing their significant impact on the effective Lagrangian.This paper explores the role of low-lying meson resonances (spin ≤ 1) in the context of chiral perturbation theory (CHPT) for pseudoscalar mesons. The authors determine the contributions of these resonances to the coupling constants of the O(p^4) effective chiral Lagrangian, which describes strong interactions at low energies. They find that these resonance contributions dominate the low-energy coupling constants, particularly for vector and axial-vector mesons. The analysis supports the vector meson dominance hypothesis, suggesting that vector mesons play a prominent role in low-energy hadronic interactions. Additionally, the authors calculate the electromagnetic pion mass difference using explicit resonance fields, demonstrating that the low-energy constant at O(e^2 p^0) is also dominated by resonance contributions. The paper concludes with a systematic treatment of all low-lying meson resonances in CHPT, emphasizing their significant impact on the effective Lagrangian.