The synthesis and processing of dense silicon-based bulk ceramic materials derived from the thermal decomposition of preceramic organosilicon polymers such as polysilazanes and polysilanes was achieved by three different routes A, B, and C. Additive-free silicon carbonitride bodies of the composition Si1.7C1.0N1.6 were produced according to route A, with up to 94% relative density by the pressureless pyrolysis of compacted infusible polysilazane powders at low processing temperatures (1000 °C). The resulting silicon carbonitride was single-phase and amorphous according to X-ray- and TEM-investigations and exhibited a low solid-phase density of 2.33 g/cm3. The maximum room temperature fracture strength of additive-free silicon carbonitride was 370 MPa. Crystallization of the as-synthesized silicon carbonitride bulk samples occurred at temperatures exceeding 1400 °C.In a second process, route B, densification of polysilazane-derived amorphous silicon carbonitride powder was achieved by liquid phase sintering with alumina and yttria as sintering additives in nitrogen atmospheres and at temperatures up to 1900 °C. This process resulted in the formation of dense polycrystalline β-Si3N4/β-SiC-composites. The average room temperature fracture strength and fracture toughness of the gas pressure sintered composite were in the range of 650 MPa and 10.2 MPa√m, respectively.Polycrystalline Si3N4/SiC-composites were obtained in a third process, route C, by the pyrolysis and subsequent sintering of α-Si3N4-powder/polysilane blends. The Si3N4-powder serves as an inert filler reducing the volume shrinkage associated with the polymer-to-ceramic transformation. Dense Si3N4 and Si2N2O bulk ceramics were formed according to route B by the liquid phase sintering of amorphous silicon nitride powder synthesized by the polysilazane pyrolysis under ammonia.