The latest generation of infrared long-baseline interferometric instruments combines high spatial resolution with spectroscopic capabilities, enabling fascinating new studies of the AU-scale circumstellar environment around young stellar objects. Here, we present recent investigations, which we conducted using the VLTI instruments AMBER and MIDI and which demonstrate these new observational possibilities. In one study, we combine near- and mid-infrared interferometry (H-/K-/N-band) to constrain the geometry and radial temperature profile of the circumstellar accretion disk around the Herbig Be star MWC 147. Using detailed radiative transfer modeling, we find strong evidence for the presence of an optically-thick inner gaseous disk. In another investigation, we used AMBER's medium spectral resolution mode (R = 1500) to study the spatial origin of the hydrogen Bry line for five Herbig Ae/Be stars, associating the line emission with different physical mechanisms, such as disk winds and magnetospheric accretion. Finally, we present AMBER H- and K-band observations of the close binary star θ1 Orionis C and illustrate the benefits of fitting wavelength-differential visibilities and closure phases. Besides yielding a high observing efficiency, this approach is also insensitive to calibration errors, induced, for instance, by fast changing atmospheric conditions.