The phenomena responsible particle fusion during selective laser sintering (SLS) have been modeled, but attempts at experimental validation have been limited to unrealistic conditions compared to those used in commercial SLS processes. We have designed, implemented, and tested an instrument that is capable of performing in situ microscopy to image particle-scale dynamics of the SLS process at realistic scan speeds, temperatures, and gas environments. The instrument uses a combination of purchased and manufactured components to capture video at length scales of ∼1 µm and timescales of ∼1 ms. Spatial and temporal calibration is performed so that the laser position can be mapped onto the recorded video. Results show that laser melting of polymer is accompanied by particle rearrangement that occurs over an extended time (∼10 ms) after the laser scans a particular location. The findings highlight the importance of performing in situ microscopy under realistic conditions for SLS.