The investigation described in this thesis is concerned with the identification of the products of isothermal transformation of austenite containing 1.44% C, at temperatures just above and just below the Ms temperature, and determination of the isothermal transformation diagram for this range of temperatures. In part A of the thesis, the literature relevant to the martensite and lowerbainite transformations is reviewed, with particular emphasis being placed on those features of each transformation which may possibly be used to identify the product. It is concluded that habit plane measurements offer the most suitable experimental means for identification of the products. In part B the experimental techniques of quantitative metallography, for habit plane measurements, and optical and electron microscopy, for study of the morphology and internal inhomogeneities of the products, are described.