The complexity, nonlinearity, and nonstationarity of biological time series typically defy quantitative description. Living systems are governed by numerous, continuously changing, interacting variables in the presence of noise. Such conditions often challenge traditional methods such as Fourier transforms. An approach for assessing such nondeterministic complexity is recurrence quantification analysis (RQA). Strategies implementing quantification of recurrences have often been successful in diagnosing changes in nonstationary signals not easily detected by traditional methods.