The aim of this chapter is to provide the reader with a simplified description of the main thermodynamic and structural characteristics of emulsions and microemulsions. The wide range of applicability of these complex fluids in several fields has been accompanied by a vast body of fundamental, structural and theoretical studies in the past decades. In this contribution we will illustrate the main issues underpinning a tailored design and comprehension of the properties of emulsion and microemulsion systems. After an introductory section, where nomenclature and definitions will be presented, we will describe in more detail the main determinants that drive phase behaviour and constitute predictive tools to assess the suitability for a given application, such as the cleaning of works of art.