Magnetic fluids such as ferrofluids and ferrofluid emulsions (nanoemulsions) are stimulus responsive materials whose physical properties can be strongly modified by an external magnetic field. Such magnetic fluids have several important technological applications. Irrespective of several studies in the past, a complete understanding of the field induced aggregation dynamics and associated magneto-optical properties in magnetic fluids is still lacking. The main objective of this work is to obtain better insight into the complex aggregation, disorder-order transitions, and the intriguing magneto-optical phenomena in magnetic fluids using the forward light scattering, back-scattering, and absorption and speckle dynamics studies. The role of aggregation parameters such as suspended nanoparticle size and temperature on structural transitions and associated magneto-optical properties in magnetic fluids are investigated. The dramatic changes in the external magnetic field induced transmitted as well as back-scattered light intensity in magnetic fluids is explained using the dipolar Mie resonances in the scattering medium. The changes in scatterer dimension and surface roughness during disorder-order transitions of suspended nano-particles to chain-like structures strongly influence the transmitted and the back-scattered speckle parameters such as intensity, contrast, degree of coherence, polarization and speckle correlation coefficient. Use of Light, X-ray & neutron scattering, ellipsometry for investigation of the aggregation dynamics in magnetic fluids, and the associated intriguing physical properties in nano colloidal systems in presence of different external stimuli (magnetic field, electric field) will be briefly discussed.