Abstract: The theoretical bases and approaches of cladistics and some specific problems that, directly or indirectly, rely on cladistic analysis for their revo- lution, are outlined and discussed. Seven sections comprise this paper: a ) the phi- losophical foundation of cladistics; b) the theoretical tenets of cladistics; c) the operational procedure of cladisties; d) three schools of classification; e) cladistics and biogeography; f) cladistics and hybrid recognition; and g) is cladistic sys- tematics a scientific theory ?   Considerations of scientific methodology involve philosophical questions. From this point, Popper'falsificationism serves a good foundation.  Popper emphasizes that all scientific knowledge is hypothetical-deductive, consisting of general statements (theories) that can never be confirmed or verified but only falsified.  The theories,  that can be tested most effectively,  are preferable. Cladistics, aiming at generating accurately expressed and strictly testable systematic hypotheses, is well compatible with this requirement.   The principles central to the cladistic theory and methodology are:  the Principle of Synapomorphy; the Principle of Strict Monophyly; and the Principle of Strict Parsimony.  The first requires forming  nested  groups  by  nesting statements about shared evolutionary novelties (synapomorphy) postulated from observed  similarities  and  is  the  primary  one.  The  second  is  mainly methodological, subject to modification and compromise. The principle of strict parsimony specifies the most preferable hypothesis (namely the one exhibiting the most congruence in the synapomorphy pattern).   The operational procedure that might be followed in formulating and testing hypotheses of the synapomorphy pattern (the cladogram itself) consists of five steps. The erections of monophyletic groups, to a greater or lesser extent, rely on the hypothesis of the previous systematic studies and is the starting point for cladistic analysis. Character analysis, which focuses on character distribution and determination of the polarities, decides the reconstructed phylogeny. A detailed dis- cussion on the methodological principles for identifying transformation sequence is presented.  Many algorithms have been designated to infer the cladogram, and are basically of parsimony techniques and Compatibility techiques. The thus yielded cladograms, with their expected pattern of congruent synapomorphies, are tests of a particular hypothesis of synapomorphy and reciprocally synapomorphies are tests of cladistic hypothesis (cladogram). Such reciprocity is a strong stimulus to profound understanding on phylogenetic process and phyletic relationships. The cladogram and the Linnaean classification have the identical logic structure and the set-membership of the two can be made isomorphic.   There are three principal approaches to biological classification : cladistics, phenetics and evolutionary classification. Cladistics is the determination of the branching pattern of evolution, and  in the context of classification, the develop- ment of nested sets based on cladograms. Phenetics is the classification by overall similarities, without regard to evolutionary considerations. Evolutionary classifica- tion attempts to consider all meaningful aspects of phylogeny and to use these for making a classification. The last approach has been done intuitively, without ex- plicit methods. An enumeration of their differences and a discussion on their rela- tive merits are presented.   Three  theoretical  approaches  have  been  proposed  for  interpreting biogeographical history: the phylogenetic theory of biogeography, classical evolu- tionary biogeography and vicariance biogeography. The former two show some similarities in that they usually look upon biogeography in terms of centers of origin and dispersal from the centers. But the first puts a strong emphasis on the construction of hypotheses about the phylogenetic relationships of the organisms in question and the subsequent inference of their geographic relationships; the second advocates a theory which does not have a precise deductive link with phylogenetic construction and often results in wildly narratative-type hypotheses. The vicariance approach de-emphasizes the concepts of centers of origin and dispersal and at- tempts to analyse distribution patterns in terms of subdivision (vicariance) of ancestral biotas.  The development of the theory of plate tectonics and its universal acceptance enormously stimulate biogeographers to look at the world's continents and oceans from a mobilist point, which, along with the establishment of the rigorous tool of the phylogenetic analysis (cladistics), profoundly reshapes the above three theories.   Hybridization and polyploidy are outstanding features of many plant groups. But hybridization, or reticulate evolution, is inconsistent with the basic concepts of cladistics which is an ever-branching pattern. Cladists have suggested several approaches. One of them analyses all the taxa by a standard cladistic procedure and closely examines the cladograms for polytomies and character conflicts that may indicate possible hybrids. Such generated hypothesis of hybridization can be corroborated or falsified by other forms of data, such as distribution, polyploidy, karyotype and pollen fertility.   There are three criteria to justify a theory to be scientific: a) whether it is a theory composed of hypotheses strictly falsifiable; b) whether it has predictive effect; and c) whether it has a explanatory value. Cladistic systematics aims at generating  cladograms,  which  are  hypotheses  of  the  nested  pattern  of synapomorphy, phylogenetic process and phyletic relationships, susceptible to testing by postulated synapomorphies. The predictive effect of systematics relies on the acceptance of hypotheses of congruence about the correlati