Contents About the Authors Symbols 1 Introduction 1.1 Linear Optical Response 1.2 Low-Energy Excitations in Condensed Matter 1.2.1 Electronic excitations 1.2.2 Vibrational excitations 1.3 Charge Transport in Solids Bibliography 2 Terahertz Technology 2.1 Generation of Coherent Terahertz Radiation 2.1.1 Terahertz lasers 2.1.2 THz generation by optical frequency conversion 2.1.3 Photoconductive switches 2.2 Phase-Resolved Detection of Terahertz Transients 2.2.1 Photoconductive antennas 2.2.2 Electro-optic sampling 2.3 Linear Terahertz Spectroscopy and Imaging Bibliography 3 Nonlinear Light-Matter Interactions 3.1 Nonlinear Optical Response in the THz Range 3.1.1 Theoretical framework 3.1.2 Approximations 3.2 Nonlinear Currents in Condensed Matter 3.2.1 Shift currents in systems lacking inversion symmetry 3.2.2 Nonlinear currents of electrons in a periodic potential 3.2.3 Nonlinear currents without real-space transfer of carriers 3.3 Quantum Coherences in Coupled Multi-Level Systems 3.4 Nonperturbative Regime of Light-Matter Interaction Bibliography 4 Methods of Nonlinear Terahertz Spectroscopy 4.1 Concepts and Experimental Implementation 4.1.1 Optical geometries for nonlinear THz experiments 4.1.2 Interaction schemes and pulse sequences 4.2 Two-Dimensional Spectroscopy 4.2.1 Experimental methods of two-dimensional THz spectroscopy 4.2.2 Data representation and analysis Bibliography 5 Nonlinear THz Spectroscopy of Condensed Matter 5.1 Low-Energy Excitations in Liquids and Solids 5.1.1 Condensed-phase molecular systems 5.1.2 Solids and nanostructures 5.2 Field-Driven Nonlinear Response and Charge Transport 5.2.1 Nonlinear THz response of softmodes in a molecular crystal 5.2.2 Polaron transport 5.2.3 Coherent high-field transport in GaAs on femtosecond time scales 5.2.4 THz-induced interband tunneling of carriers 5.2.5 Shift current in LiNbO3 5.2.6 Nonlinear transport in graphene 5.3 Conclusions and Outlook Bibliography