In recent years, devices based on quantum dots formed by gating a two-dimensional electron gas have found many applications, e.g., in charge sensing and quantum information. New types of quantum dot devices, based on nanowires, carbon nanotubes, and even single molecules, have further interesting properties, such as meta-stable states and non-trivial spin or mechanical degrees of freedom. A central problem is that such devices cannot be made in a completely reproducible or controlled way. Spectroscopic investigation of devices is therefore of paramount importance. Standard spectroscopic techniques typically cannot be used to investigate the quantum dots in situ, and the only remaining option is to measure the current through the device as function of the applied voltages and use this to extract information about the dot. Such transport spectroscopy is the topic of my talk. I will introduce a theory for transport through strongly interacting dots, which goes beyond lowest order in the tunnel coupling, and show that this theory predicts a new type of transport resonance, originating from coherent tunneling of electron pairs. Furthermore, I will discuss some recent experiments on different types of dots. Interpreting experimental transport spectra is often a tricky business, but I will show that a direct comparison to model calculations can help to uncover the underlying physics.