This study investigated students' understanding and application of resonance structures in organic chemistry. The findings revealed students' challenges when reasoning about resonance and its implications for acidity, chemical shift, and geometry. Despite exposure to resonance structures in general chemistry and use throughout organic chemistry, resonance was mentioned by only 5 of the 12 students interviewed. The study uncovered how 580 students often relied on resources related to s-character, inductive effects, and electronegativity. Furthermore, some students made errors in applying formal charges or violated the octet rule when drawing resonance structures. These findings have important implications for teaching practices -particularly when first introducing resonance structures.