Crystalline silicon wafer solar cells have been dominating the photovoltaic market so far due to the availability and stability of c-Si and the decades of Si technology development. However, without new ways to improve the conversion efficiencies further significant cost reductions will be difficult and the c-Si technology will not be able to maintain its dominant role. In the SiTaSol project we want to increase conversion efficiencies of c-Si solar cells to 30 % by combining it with III-V top absorbers. Such a tandem solar cell will result in huge savings of land area and material consumption for photovoltaic electricity generation and offers clear advantages compared to today’s products. The III-V/Si tandem cell with an active Si bottom junction with one front and back contact is a drop-in-replacement for today’s Si flat plate terrestrial PV. To make this technology cost competitive, the additional costs for the 2-5 µm Ga(In)AsP epitaxy and processing must remain below 1 €/wafer to enable module costs <0.5 €/Watt-peak. It is the intention of the SiTaSol project to evaluate processes which can meet this challenging cost target and to proof that such a solar cell can be produced in large scale. Key priorities are focused on the development of a new growth reactor with efficient use of the precursor gases, enhanced waste treatment, recycling of metals and low cost preparation of the c-Si growth substrate. High performance devices will be demonstrated in an industrial relevant environment. The project SiTaSol approaches these challenges with a strong industrial perspective and brings together some of the most well-known European partners in the field of Si PV and III-V compound semiconductors. Furthermore SiTaSol will support the competitiveness of the European industry by providing innovative solutions for lowering manufacturing costs of III-V materials which are essential in today’s electronic products including laptops, photonic sensors and light emitting diodes.