Multiphase flow is a subject distinct, and much more difficult than single phase fluid mechanics and heat transfer. This is because while the starting principles are the same (i.e., the conservation of mass, momentum, and energy), the presence of coupled fluid motions, interacting through the unspecified geometry of the interfaces, requires a whole new scientific methodology for understanding, as well as a whole new engineering apparatus for practical applications. The simplest manifestations of these, in pipes, have been classified as "flow regimes," and they adapt to changes imposed externally. Even simple flow cases reveal multiphase flows as a class of non-linear adaptive systems, an immense frontier of science in the 21st century (Science, 1999). Although multiphase flows are ubiquitous in science, technology, and the environment, their complexity outmatched the "ownership" needed for a systematic and fundamental advancement of the field towards becoming a "discipline." (ImuST 2002, research Proposal to DOE NERI and US NRC) In the past five years, the Laboratory for Fluid Dynamics and Thermodynamics (LFDT) at Ljubljana University played well documented role in new development trends of multiphase science. The involvement can be traced in the open literature, citations that amount close to 200 as well as activities in the IMuST (Institute of Multifluid Science and Technology) in the US, in the ICeM (International Information Center for Multiphase Flow) in Japan and in the EMSI (European Multiphase Systems Institute). The research group has been involved in six European projects: PREVERO (RTD) and HMTMIC RTN) of the 5th frame program, COST, ULTRAWAT, wlDelft and Surgenet and 3 bilateral projects with Germany, France and the US. Main contributions can be assorted into two areas of interest: 1.1Interfacial structures and evolutionary properties of bubbly flow 1.2Complex two-phase systems and 2. Development of new numerical models for solid-liquid sistems.