The chemistry of heterocyclic compounds is an important part of applied organic chemistry. The abundance of heterocyclic compounds in nature and the wide range of industrial applications these compounds are useful in justify the great interest in them from the side of researchers. From medicine to energy heterocyclic compounds are present with endless potentials. In this book a broad work in the field of heterocyclic chemistry were collected. These included five and six-membered rings either simple or condensed. Five- membered heteroycles included derivatives of oxazoles, thiazoles, pyrazoles, triazoles, imidazolines, indoles, selenolopyridazines and selenolopyridines. Six-membered heterocyclic derivatives included pyranes, thiopyranes, phenoxazines, pyridines, pyrimidines, pyrazines, triazines and 1,6-diazanaphthalenes. These compounds have potential industrial applications as dye intermediates, photographic ingredients and corrosion inhibitors. The expected free radical scavenger potentials of selenolopyridazines and selenolopyridines open the way of their medicinal deployment in disorders where oxidative stress plays a role in pathogenesis as in ischemic brain and heart insults.