During the course of the present thesis, I have studied the thermal and light-induced spin switching dynamics in Fe(II) spin-crossover complexes both in bulk single-crystal form and in nanoparticles form. In the bulk single-crystal form, I have worked with three different classes of spin-crossover complexes, (a) the 2D coordination polymeric network of [Fe(bbtr)3]X2 (where bbtr = [1,4-di (1,2,3-triazol-1-yl) butane] and X= ClO4-, BF4-) and their diluted mixed crystals in the Zn-host matrices; (b) the Fe(II) Complexes of 2,2′-Biimidazole and their alkylated derivatives and (c) the Fe(II) complex of [Fe(6-mepy)3tren](PF6)2 (where 6-mepytren = tris{4-[(6-methyl)-2-pyridyl]-3-aza-butenyl}amine) and its diluted mixed crystals in the Zn-host matrices. Moreover, crystalline and amorphous nanoparticles of [Fe(6-mepy)3tren](PF6)2 have also been studied. The aim of the present thesis is therefore twofold: (a) understand the physics of cooperative effects and establish the limits of cooperativity in Fe(II) spin-crossover complexes, (b) understand the physical basis of the structure-function correlations in Fe(II) spin-crossover complexes.