Cross-sectional scanning tunneling microscopy on semiconductor structures is evolving into a technique to analyze structural, chemical, and electronic properties on the atomic and nanometer scale in all spatial dimensions, in particular, in the lateral and in-depth spatial dimensions of the structure. This technique has been used on the ultrahigh vacuum cleaved (110) plane of (001)-grown AlGaAs/GaAs heterostructures. Measurements of (i) interface roughness, alloy fluctuations, and ordering; (ii) the variation of electronic properties over an interface as well as fluctuations within the alloy; and (iii) the distribution of individual dopant sites are reported on.