A typical microstructure in a low-alloyed steel featuring the so-called TRIP-effect, short for “transformation-induced plasticity”, consists of ferrite, bainite and metastable austenite. These austenite grains transform to martensite during plastic deformation. The accompanied volume expansion leads to an improved strain hardening rate, compared to steels without TRIP-effect. Aim of this diploma thesis was to observe and characterize the process of phase transformation at a microstructural level, which is very difficult because of the small grain sizes. To do this, modern experimental techniques like digital image correlation (DIC) and electron backscatter diffraction were employed to measure local strains and the crystal structure, respectively. Tensile tests were carried out ex-situ and in-situ inside a high-resolution scanning electron microscope to obtain highly magnified pictures of the microstructure at different stages of deformation. Processing of these pictures by DIC allowed us to study the evolution of the local strain fields during the tensile test. After improving some experimental details, it was possible to examine the deformation behaviour of a few single crystals, and we succeeded for the first time in directly observing the strain induced transformation of single austenitic grains. Transformed grains show characteristic, new surface features. In addition, local TRIP-strains could be directly quantified for the first time.