Two approaches have been used to investigate magnetization reversal in sub-micron ferromagnetic particles. First, a technique was developed which enables one to measure the remanent coercivity of an individual particle and to examine the particle in-situ so that its physical structure and orientation to the applied field can be determined. This technique is based upon the use of the Foucault mode of Lorentz microscopy to detect the polarity of an individual particle's magnetic moment, and the ability to repeatedly locate the same particle on the TEM sample after the sample has been removed from and returned to the electron microscope. Second, a method was developed for preparing dispersions of particles which have been coated with multiple layers of 10 nm silica spheres and which contain a large number of mono-dispersed particles. This enables one to prepare a series of assemblies in which the inter-particle separation, and hence the strength of the particle interactions, is varied. These complimentary techniques were applied to $\gamma$-Fe$\sb2$O$\sb3$ particles which are nearly ellipsoidal in shape and which have a narrow distribution of sizes and morphologies, thus allowing us to obtain information on a model system for studying magnetization reversal in ferromagnetic particles. Switching measurements on 6 isolated, nearly-ellipsoidal particles having a diameter of 0.065 $\mu$m and an aspect ratio of 4.6 yielded remanent coercivities in the range 760-1240 G, with 4 of the 6 having remanent coercivities above 950 G. A weakly-interacting assembly of silica-coated particles was prepared in which 67% of the particles are mono-dispersed, with an interparticle separation of 0.18 $\mu$m. The mono-dispersed particles comprise the upper 67% of the assembly's switching field distribution and have switching fields in the range 732-1400G, in agreement with the measurements on single particles. The fact that the weakly-interacting assembly has a squareness of 0.95 indicates that the particles are single-domain and allows a lower bound of 1.3 $\times$ 10$\sp{-7}$ erg/cm to be put on the exchange constant for $\gamma$-Fe$\sb2$O$\sb3$.