questions,but was able to approach them in new ways.with the help of intellectual tools that had not previously been available:in particular,a clear understanding of the nature of recursive processes,generative procedures that can characterize an infinity of objects (in this case,expressions of L)with finite means the mechanisms of L).As soon as the inquiry was seriously undertaken,it was discovered that traditional grammars and dictionaries.no matter how rich and detailed,did not address central questions about linguis ic expressions.They basically provide "hints"that can be used by somcone cquipped with FL and some of its states,but leave the nature of these systems unexamined.Very quickly,vast ranges of new phenomena were discovered,along with new problems,and sometimes at least partial answers. It was recognized very soon that there is a serious tension between the scarch for descziptive and for explanatory adequacy.The former appears to lead to very intricate rule systems,varying among languages and among constructions o:a particular language.But this cannot be correct,since each language is attained with a common FL on the basis of PLD providing l.ttle information about these rules and constructions. The dilemma led to efforts to discover general propertics of rule systems that can be extracted from particular grammars and attributed to UG,leaving a residue simple enough to be attainable on the basis of PLD.About 25 years ago,these efforts converged in the so-called "principles and parameters' (P&P)approach,which was a radical break from traditional ways of looking at language.The P&P approach dispenses with the rules and constructions that constituted the framework for traditional grammar,and were taken over,pretty much,in early generative grammar.The relative clauses of Hungarian and verb phrases of Japanese exist,but as taxonomic artifacts, rather like "terrestrial mammal"or "creature that flies.The rules for forming them are decomposed into principles of UG that apply to a wide variety of traditional constructions.A particular language L is determined by fixing the values of a finite number of "parameters"of S-0:Do heads of phrases precede or follow their complements?Can certain categories be null (lacking phonetic realization)?Etc.The parameters must be simple enough for values to be set on the basis of restricicd and easily obtained data. Language acquisition is the process of fixing these values.The parameters can be thought of as "atoms"of language,to borrow Mark Baker's metaphor. Each human language is an arrangement of these atoms,determined by assigning values to the parameters.The fixed principles are available for constructing expressions however the atoms are arranged in a particular I- language.A major goal of research,then,is to discover something like a "periodic table"that will explair.why only a very small fraction of imaginable linguistic systems appear to be instantiated,and attainable in the normal way. Note that the P&P approach is a program,not a specific theory;it .s a F18