The investigation of the system In-Ch-X (Ch = S, Se, X = Cl, Br), considering the application of the concept of structural building blocks makes the leitmotif of this monograph along with the syntheses, structural characterization and the thermal behavior of four new representatives of the series of mixed valence chalcogenide halides of indium: In5S5Cl, In5Se5Cl, In5S5Br, In5Se5Br. At the first glance, the similarity between their sum formulas gives the illusion that all of them would belong to a single structure type. Detailed single crystal XRD analyses, combined with HRTEM images revealed the existence of two significantly different structure types: the chloride-type In5Ch5Cl and the bromide-type In5Ch5Br (Ch = S, Se). Their bulk structures represent an average of complex fine structures interlaced with a variety of nanostructural defects, especially visible for In5Ch5Cl (Ch = S, Se). Attempts of substitution of one or more indium positions in them by similar species brought to light the existence of two quaternary isostructural substitutes, containing thallium, i.e. TlIn4S5Cl and TlIn4Se5Cl. Their thermal behavior/degradation is thoroughly elucidated by HT-powder XRD & DTA.