Biomembranes consist of a complex mixture of a large number of lipids and proteins. In such mixtures, microscopic domains and macroscopically separated phases may exist. Here, we discuss phase behavior and domains formation of binary lipid mixtures. We show that the domain formation is accompanied by large fluctuations at the domain boundaries, resulting in altered physical properties at the boundaries, for instance in a pronounced increase of the elastic constants. Therefore we argue that the physics of the membrane depends on the overall length scale of its domains interfaces. We present here confocal microscopy images, calorimetric melting profiles and Monte-Carlo simulations to understand the factors that determine domain formation, their sizes and the role of the domain interfaces.