The compounds that contain heterocyclic rings in their structures present diverse properties according to the number and nature of the different substitute to which they are united. Thus, these derivatives are broadly studied from different points of view. This book presents important studies related to the structures, vibrational spectra and descriptors of two series of heterocyclic derivatives; on one hand, some derivatives are of importance for the health and, on the other hand, others are of environmental importance. Chapters 1 to 6 are oriented towards the first series of derivatives broadly used in the pharmacological industry for human health. From Chapters 7 to 9, the studies are oriented toward the properties of the hexaclhorobenzene, 2,4 dichlorophenoxy acetic acid and sulfonylureas derivatives where the sulfonylureas are herbicides of low dosage, low mammalian toxicity, high selectivity, and benign environmental activity while on the contrary, hexaclhorobenzene and 2,4 dichlorophenoxy acetic acid present high toxicities.