This paper by Hans Schmid reviews the domain aspects of multi-ferroic materials, which exhibit simultaneous ferroelectricity, ferromagnetism, and ferroelasticity in the same phase. The focus is on boracite crystal family, with limited data on perovskite and other classes of materials. Polarized light microscopy is highlighted as an essential tool for studying multi-ferroics, especially when ferroelasticity is involved. The paper discusses the requirements for these properties to occur simultaneously, including structural and symmetry requirements. It also explores the complexity of domain structures and the challenges in experimental studies, such as the determination of the magnetic point group. The paper provides examples of well-established multi-ferroic materials and discusses the limitations and experimental strategies for studying complex multi-ferroics.This paper by Hans Schmid reviews the domain aspects of multi-ferroic materials, which exhibit simultaneous ferroelectricity, ferromagnetism, and ferroelasticity in the same phase. The focus is on boracite crystal family, with limited data on perovskite and other classes of materials. Polarized light microscopy is highlighted as an essential tool for studying multi-ferroics, especially when ferroelasticity is involved. The paper discusses the requirements for these properties to occur simultaneously, including structural and symmetry requirements. It also explores the complexity of domain structures and the challenges in experimental studies, such as the determination of the magnetic point group. The paper provides examples of well-established multi-ferroic materials and discusses the limitations and experimental strategies for studying complex multi-ferroics.