This lecture is about the concepts of modern laser spectroscopy. An introduction to lasers, in particular those generating ultra-short optical pulses, is given. We then discuss nonlinear optical phenomena that can be observed thanks to the high peak intensities of short laser pulses, as well as their applications. This leads to the description of various time-resolved spectroscopic techniques, including fluorescence up-conversion, pump-probe transient absorption and terahertz spectroscopy, which will be illustrated with case studies.