This review presents the synthesis, characterization techniques, processing and potential applications of silicon-based ceramic materials derived from organosilicon polymers. The Si-ceramics are prepared by thermolysis of molecular precursors. The influence of the initial molecular structure of the precursor on the properties of the final ceramic material and its applications is discussed. The thermolytic decomposition of suitable Si-based polymers provides materials which are denoted as polymer-derived ceramics (PDCs). In particular, this procedure is a promising method for the preparation of ternary and multinary silicon-based ceramics in the system SiCNO. There is no other synthetic approach known to produce e.g. SiCO or SiCN based ceramics. In the case of PDCs route, common preceramic polymers are poly(organosilazanes), poly(organosilylcarbodiimides) and poly(organosiloxanes). One basic advantage of the PDC route is that the materials can be easily shaped in form of fibers, layers or bulk composite materials by applying processing techniques established in the plastic industry. The PDCs in general exhibit enhanced thermomechanical properties, i.e., temperature stabilities up to approximately 1500°C. Recent investigations have shown that in some cases the high temperature stability in terms of decomposition and/or crystallization can be increased even up to 2000°C if the preceramic polymer contains some amount of boron. The composition and microstructure of the PDC are a result of the molecular structure of the preceramic polymer. Therefore, the observed differences in the macroscopic properties are also closely related to the variation of composition and solid state structure of these materials.