Nonlinear terahertz (THz) spectroscopy gives insight into elementary
excitations and high-field charge transport in semiconductors. Strong THz transients with field amplitudes of up to megavolts/cm serve as a driving field for free carriers and the resulting transport behavior is directly inferred
from the field radiated by the moving charges. New results on ballistic transport of electrons in bulk GaAs and the transition from a ballistic into a drift-like regime of transport will be discussed. This will be followed by an outlook which addresses the potential of two-dimensional THz spectroscopy for unraveling couplings between different elementary excitations.