In this tutorial review in the thematic series "Acoustofluidics", we discuss the implementation and practice of optical microscopy in acoustofluidic micro-devices. Examples are given from imaging of acoustophoretic manipulation of particles and cells in microfluidic channels, but most of the discussion is applicable to imaging in any lab-on-a-chip device. The discussion includes basic principles of optical microscopy, different microscopy modes and applications, and design criteria for micro-devices compatible with basic, as well as advanced, optical microscopy.