This chapter discusses real‐time observation of spin‐transitions by optical microscopy (OM). The physical features revealed by OM of spin‐transition crystals are consistent with well‐established observations on (first‐order) phase transitions in organic crystals. That is, they evidence a nucleation and growth process for the spin transition. The authors, therefore, reached the crucial conclusion that the spin‐transition obeys a multiscale, stress‐driven, nucleation and growth process. In the present case of switchable molecular solids, due to their thermo‐ and photochromic properties, OM can be used as a local colorimetric tool. The change in optical density (OD) associated with a specific absorption band enables local measurements of the HS fraction of the system. The use of cross‐polarizer geometry may reveal the onset of a symmetry change concomitant to the spin change, thus allowing an investigation of the interplay between the spin and structural changes. Cross‐polarizer geometry may also reveal the presence of internal stresses.