Laser ablation (LA) is one of simple and versatile techniques for synthesis of nanoparticles [1]. The major advantages of this method are in its green character and in the possibility of a control over particle size. These advantages make LA a unique tool for nanoparticle synthesis. In this study, we examine the mechanisms involved in nanoparticle formation by laser ablation in gases and in liquids. First, we consider the very early ablation stage providing initial conditions for much longer plume expansion processes. In the case of femtosecond and picosecond laser ablation, the initial population of primary nanoparticles are shown to be formed at this stage. Then, when either ambient gases or liquids are present, the ablated flow suffers a background pressure. The dynamics of the laser plume expansion is changed revealing the formation of shock waves if the background pressure is high enough [2]. As a result, the ablated material is compressed and a part of it becomes supersaturated [3].When ablation takes place in liquids a cavitation bubble is also formed for larger laser fluences. The second population of primary nanoparticles is formed at this stage. Finally, during a much longer later stage, nanoparticle aggregation/fragmentation enters into play leading to the formation of the secondary particles. Based on numerical modelling we shed light on the above mechanisms by using different numerical approaches, such as molecular dynamics, DSMC [4], numerical hydrodynamics [3] and analytical analysis [2]. Calculations are performed mostly for metallic targets and for carbon under different background conditions. The obtained results explain recent experimental findings and help to predict the role of the experimental parameters. The performed analysis thus indicates ways of a control over nanoparticle synthesis. [1]T. E. Itina, N. Bouflous, J. Hermann, P. Delaporte, SPIE Proc., 8414, DOI: 10.1117 /12.923129 (2011). [2]T. E. Itina and A. Voloshko, Appl. Phys. B, DOI: 10.1007/s00340-013-5490-6 (2013). [3]M. E. Povarnitsyn, T. E. Itina, P. Levashov, K. Khishchenko, Phys. Chem. Chem. Phys., 15, 3108-3114, DOI: 10.1039/C2CP42650A (2013) [4]T. E. Itina, K. Gouriet, L. V. Zhigilei, S. Noel, J. Hermann, and M. Sentis, Appl. Surf. Sci. , 253, 7656-7661, DOI: 10.1016/j.apsusc.2007.02.034 (2007)