The relevant structural size of self-assembled colloidal systems is typically in the range of 1-500 nm, which is a size range very well suited to be probed by small-angle neutron scattering (SANS). In addition, many interesting systems are fluid with water as (main) solvent, while the colloidal system is of organic nature. Accordingly one can achieve high contrast by using D2O as solvent and also can easily change the contrast by employing mixtures of H2O and D2O. All these aspects make SANS a tool of unique power to study the mesoscopic structure of self-assembled systems and to gain detailed structural insights over a large range size range. In this talk we will look at various examples of self-assembled colloidal systems, where SANS allows to deduce a structural picture that allows to understand the properties of the systems studied. One example are nonionic surfactants where the systematic variation of their hydrophilic head group allows to tune their micellar structures and in particular their interactions, which control the formation of liquid crystalline phases at high concentration. In this fashion one can then suppress the formation of gel-like liquid crystalline phases thereby allowing to process these surfactants at much higher concentration than usually possible. In another example we look at complexes of chitosan with oppositely charged surfactant. By variation of pH and mixing ratio one can change largely the size and structure of the formed complexes. This pH sensitivity then can be exploited for pH-dependent separation processes. Finally SANS was also used to follow the layer-by-layer process of building up multi-walled amphiphilic nanotubes, thereby allowing to characterize the systematic build-up of these interesting structures. In summary, SANS is an investigative tool that can be employed for studying a large range of colloidal systems and deliver valuable information to understand their properties and tailor them accordingly.