"Liquid-vapor phase transitions and boiling are omnipresent in science and technology, but, as far as basic understanding of the hydrodynamics, these phenomena remain ""terra incognita''. The objective of the proposed work is to achieve a fundamental understanding of the fluid dynamics and heat transfer of the liquid-vapor phase transition - in particular of boiling - both on a micro- and on a macro-scale, through experiments under well-defined and controlled conditions, accompanied by theoretical and numerical modeling. Up to now ""boiling'' has been nearly exclusively an engineering subject. We want to change this and make it a physics subject as we are convinced that boiling involves very interesting and practically relevant physics still in need of understanding.On the micro-scale the planned experiments include nucleation studies of individual and interacting vapor bubbles on superheated, geometrically and chemically micro- and nano-structured surfaces. In the bulk of the flow, nucleation will be achieved through laser heating, through local pressure gradients, and through acoustically triggered vaporization of metastable perfluorcarbon nanodroplets in a superheated liquid. The vapor bubbles will be monitored with ultra-high-speed digital imaging, micro particle velocimetry, infrared thermography, and heat flux measurements. On the theoretical side we will use molecular dynamics simulations and the level-set method.On the macro-scale the focus is on closed boiling turbulent flows, namely Rayleigh-Benard and Taylor-Couette flow. We will measure how the vapor bubble formation affects global quantities such as the heat flux and the angular momentum flux and thus the drag, and local flow properties such as the vapor bubble concentration. The numerical simulations, with one generic code for both geometries, will be based on discrete particle models."