Both the crystalline Si and the second-generation Si thin film solar cells dominate the current photovoltaic (PV) industry. Third-generation nanotechnology approaches aim to decrease costs to well below the $1.0/W level of second-generation PVs to $0.5/W, potentially to $0.2/W or better, by significantly increasing efficiencies but maintaining the economic and environmental cost advantages of thin-film deposition techniques. This one-hour talk will present the recent achievements in Si nanocrystalline and nanowire solar cells, since the future PV industry may belong to these nanostructured Si solar cells.Hydrogenated nanocrystalline silicon (nc-Si:H) is a mixed material comprised of an amorphous phase and crystalline grains with a wide range of crystalline volume fraction. Compared with the bulk silicon, strong optical absorption and high photocurrent are found in nc-Si:H thin films and attributed to the enhancement of the optical absorption cross section and good carrier conductivity in the nanometer grains. In comparison with the amorphous silicon thin film solar cells, there is very weak light- and current-induced degradation caused by the Staebler-Wronski effect in nc-Si:H thin film solar cells. Small grains and intermediate range order may provide a better grain boundary passivation and also improve the cell stability.We will present two schemes of three kinds of novel nc-Si:H thin film solar cells on the basis of the fact that the prevention of hot carrier cooling via phonon emission may ultimately yield meaningful efficiency gains for solar photon conversion: (1) Tandem scheme for triple-junction nc-Si:H solar cells (band gap is tuned by the dot size) with poly-Si (1.1eV) thin films, which can be maturely grown on glass substrates. (2) Hot carrier scheme utilizing the hot carriers before they relax to the band edge via phonon emission: (i) Hot carrier transport through minibands. The observed minibands in nc-Si:H thin films could be expected to slow the carrier cooling and permit the transport and collection of hot carriers at the respective p and n contacts to produce a higher photopotential. (ii) Multiple exciton generation (MEG). Highly efficient MEG effect in colloidal Si nanocrystals has been reported at lower photon energies in the visible region, which opens the possibility to expand to nc-Si:H films for MEG cells with increasing power conversion efficiency.On the other hand, nanowires can be used as efficient electrical pathways of the generated charge carriers due to high electrical mobility, especially at longer wavelengths. Nanowires can also increase the likelihood that all photogenerated electrons have a direct connection to the collection electrode. Recent theoretical and experimental work has demonstrated that the reflectance of nanowires is significantly lower than the film in the entire spectral range due to the reduced density, which can be achieved without specially designed antireflection coatings. The talk will also present (1) the preliminary application of Si nanowire arrays in planar Si solar cells, (2) the radial Si nanowire solar cell concept for use of inexpensive candidate materials (e.g., poly-Si and UMG-Si) in PV applications, and (3) recent achievements of single nanowire Si solar cells.