Spin transitions solids [1-3] are switchable inorganic compounds, between two molecular states in thermodynamic competition: a diamagnetic low-spin (LS) state and a paramagnetic high-spin (HS) state. The spin state can be switched by temperature, pressure or irradiation in the visible range [1-5]. The relevant parameter which monitors the macroscopic state of the solid is the HS fraction noted nHS, i.e. the relative number of molecules in the HS state. In strongly interacting systems, the thermal spin switching occurs abruptly at T1/2, which corresponds to the transitions temperature where 50% of HS molecules are present. Each compound has a well defined T1/2. Applying pressure induces a clear shift from HS to LS state. This behaviour is the direct consequence of the lower volume for the LS molecule. Each compound has a well defined pressure value P1/2 to convert from HS to LS state. In both cases the change from LS to HS is, in the spin crossover compounds proposed, easily detectable by optical means because in many cases colour changes from deep purple to white. In this contribution we discuss potential use of spin crossover compounds as temperature and pressure sensors.