The studyofthehealingpowerofchemicalcompounds, present into the known natural active principles and its subsequent use, shall be seen initially as a pseudoscientificprocedure, acontinuationoftheChineseandArabianoccultism, whichwasbased inthepercentagecontentoffire, air, earthandwater, aswellas ontheassociatedqualities: hot-cold, humid-dry. . ., whichthematterwassupposed to be formed by. Themethod, possessingroots in Hippocrates, Dioscoridesand Galen was studied, described and polished by Avicenna, Averrot s, Bacon and Villanova inthe MiddleAge. Italsowasappearingasastudyconstantduringthe renaissance and after. On the other hand, the birthofchemistry asascientific offspring from alchemy propitiated alternative ways ofknowledge in order to solve the same problem. In this manner, in the past century, approximately hundred years from now, Sylvester proposed the first molecular description in numerical discrete form, employing ideas which even in present times can be associated withinthesocalledmoleculartopology. Sylvester'stopologicalmodel can be considered the seed allowing the originofthis big tree, which is now knownastheoreticalchemistry. . During all the past time from the first topological modelofSylvester up to now, the proliferationofnumerical parameters to describe molecular structures has not ceased to grow larger. Some ofthese parameters have played a very important role for the understandingofthe organic molecules behavior and, by extension, for the comprehension and evaluation of their physical as well as biologicalproperties. InthemindofeveryspecialistaretheHammett'scr, theTaft constantsor the octanol-water partition coefficient. Other numerical parameters, suchasthosederivedfromthemodemtopologicalmolecularrepresentationarein aprocessofconstantrevisionandgrowing. Thus, theHosoyaandRandicindices, ortheKier'sconnectivities, amongseveralnotsowellknownnumericaldataare usual reference descriptors. They are putatthe researchers' disposition, andare easily deducible from any molecular representation in form ofordered setsof numerical figures. All ofthem are profusely studied and employed in present times. The main idea consists into the useofthese numerical data in orderto obtaininformationonthemoleculartrendstopossessoracquirecertainproperties and, even better than this, to determine in which degree or intensity molecules presenteverything.

The aim of this text is to provide reviews and monographs on topics involving molecular similarity, ranging from the fundamental physical properties underlying molecular behaviour to applications in industrially important fields such as pharmaceutical drug design and molecular engineering. The editors hope that this series will encourage new ideas and approaches, help to systematize the rapidly accumulating new chemical information, and make chemistry better understood and better applied.