  The Chapter starts with introductory information about quantitative
linguistics notions, like rank--frequency dependence, Zipf's law, frequency
spectra, etc. Similarities in distributions of words in texts with level
occupation in quantum ensembles hint at a superficial analogy with statistical
physics. This enables one to define various parameters for texts based on this
physical analogy, including "temperature", "chemical potential", entropy, and
some others. Such parameters provide a set of variables to classify texts
serving as an example of complex systems. Moreover, texts are perhaps the
easiest complex systems to collect and analyze.
  Similar approaches can be developed to study, for instance, genomes due to
well-known linguistic analogies. We consider a couple of approaches to define
nucleotide sequences in mitochondrial DNAs and viral RNAs and demonstrate their
possible application as an auxiliary tool for comparative analysis of genomes.
  Finally, we discuss entropy as one of the parameters, which can be easily
computed from rank--frequency dependences. Being a discriminating parameter in
some problems of classification of complex systems, entropy can be given a
proper interpretation only in a limited class of problems. Its overall role and
significance remain an open issue so far.
