Model theory is a branch of mathematical logic which deals with the relationship between formal logical ********s (e.g. first order logic, or variants such as continuous logic) and mathematical objects (e.g. groups, or Banach spaces). It analyses mathematical structures through the properties of the category of its definable sets. Significant early applications of model theory include Tarski's decidability results in the 1920s (algebraically closed fields, real closed fields), and in the 1960s the well-known Ax-Kochen/Ershov results on the model theory of Henselian valued fields.