Heterocycles are cyclic compounds that contain at least one atom in the ring that is not a carbon atom. These non-carbon atoms can include elements like nitrogen, oxygen, or sulfur, which contribute to the unique chemical properties of heterocycles. They are widely found in various natural products, pharmaceuticals, and agrochemicals, making them important in both organic chemistry and medicinal chemistry. Common examples of heterocycles include pyridine, furan, and pyrimidine. These compounds play crucial roles in biological systems and are often used as building blocks in the synthesis of more complex molecules. Their diverse structures allow for a wide range of applications in drug development and materials science.