A level set technique for interface tracking is presented, both for the continuum surface force formulation and the ghost fluid method approach. A projection method is used to solve incompressible Navier-Stokes equations that are coupled to a transport equation for the level set function, defined as the algebraic distance to interface. Results are presented for head-on droplet collisions in coalescence and reflexive regimes with a 2D axi-symmetric code, and for an off-center droplet collision in a separation regime for a large impact parameter with 3D code. Simulations provided realistic and various droplet collision behaviors and they correspond to experimental observations. (c) 2005 Elsevier Ltd. All rights reserved.