<p> Single molecule magnets (SMMs), such as Mn₁₂-acetate, are composed of transition metal ions and consists of identical molecules with large ground-state spin (<i>S</i> = 10) and a strong uniaxial anisotropy (65 K). Below about 3 K, Mn₁₂-acetate exhibits magnetic hysteresis with steps at specific values of longitudinal magnetic field due to resonant quantum tunneling between spin up and down projections along the easy axis. The intermolecular exchange interactions between spins on molecules are quite small and spins are considered to be independent and non-interacting. </p><p> However, the molecules do interact with each other both through magnetic dipolar interactions and through the lattice (e.g. phonons). I have investigated collective effects in SMMs due to these intermolecular interactions. In the thesis I will present experiments that explored magnetic ordering due to magnetic dipole interactions in Mn₁₂-acetate and Mn₁₂-acetate-MeOH. I will also present exper- iments on the onset of magnetic de agration in Mn₁₂-acetate due to a thermal instability.</p><p> The magnetic ordering studies involved investigating the effect of transverse fields on the susceptibility of single crystals of Mn₁₂-acetate and Mn₁₂-acetate- MeOH. Transverse fields increase quantum spin uctuations that suppress long- range order. However, the suppression of the Curie temperature by transverse fields in Mn₁₂-acetate is far more rapid than predicted by the Transverse-Field Ising Ferromagnetic Model (TFIFM) and instead agrees with the predictions of the Random-Field Ising Ferromagnet Model. It appears that solvent disorder in Mn₁₂-acetate gives rise to a distribution of random-fields that further suppress long-range order. Subsequent studies on Mn₁₂-acetate-MeOH, with the same spin and similar lattice constants but without solvent disorder as Mn₁₂-acetate, agrees with the TFIFM.</p><p> The magnetic de agration studies involved studying the instability that leads to the ignition of magnetic deflagration in a thermally driven Mn₁₂-acetate crystal. When spins prepared in a metastable state reverse, Zeeman energy is released that diffuses away. In some circumstances, the heat released cannot be compensated by thermal diffusion, resulting in an instability that gives rise to a front of rapidly reversing spins traveling through the crystal. We observed a sharp crossover from relaxation driven by heat diffusion to a self-sustained reversal front that propagates at a constant subsonic speed.</p>