By adding of Nb, V or Ti into low alloyed steels an increase of the strength and toughness occurs. These elements are called micro-alloying elements, because they are alloyed in small amounts lower than 0.1wt% whereupon the technological properties, i.e. weldability, formability, are not negatively influenced. The improvement of common steels with micro-alloying elements has been established since 1930. For manufactures the effects of the micro-alloying elements were discovered in the 1950’s. However, micro-alloyed steels have been investigated for more than 80 years. But the increasing requirements on micro-alloyed steels and also to predict the microstructure by means of thermodynamic and thermo-kinetic calculations it is important to understand the microstructural influences and changes in more detail. The aim of this doctoral thesis was to investigate the precipitation behaviour of 3 different micro-alloyed steels. One steel is alloyed with V, one with Nb and one contains Nb, V and Ti. The analyses were mainly by transmission electron microscopy and atom probe tomography. The investigations of the Nb-V-Ti steel shows that in austenite strain induced precipitation of (NbVTi)(CN) occur. They consist mainly of Nb and V and are slightly enriched in Ti. The C to N ratio of this carbonitrides is nearly 1:1. The precipitation behaviour of the Nb in steel was determined in austenite and ferrite. These examinations reveal that Nb carbonitrides in ferrite grow faster than in austenite. It was assumed that happens because of the higher diffusivity of Nb in ferrite and the lower solubility of Nb in the ferrite than in austenite. The evolution of the carbonitrides is in both phases similar. In the early stages the carbonitrides are N rich and due to depletion of N in solid solution C enriches at the particles. The formation of interphase and strain induced precipitates in ferrite was investigated in the V micro-alloyed steel. The investigations shows that interphase precipitates are enriched in Mn and C compared to randomly distributed V precipitates. The evolution of strain induced precipitates was simulated by MatCalc simulations. The results of the simulations show that MatCalc simulations are a useful tool to predict the precipitation behaviour of V particles.