This dissertation presents a systematic study on five series of spinel ferrites. Three series of spinel ferrites, namely, NiY-ferrites (NiY2xFe2-2xO4, x = 0.0 – 0.12, step: 0.02), MgY-ferrites (MgY2xFe2-2xO4, x = 0.0 – 0.12, step: 0.02) and NiZnY-ferrites (Ni0.6Zn0.4Y2xFe2-2xO4, x = 0.0 - 0.1, step: 0.02) were fabricated in a polycrystalline form by double sintering ceramic method. Two series of CoZnY-ferrites (Co1-xZnxY0.15Fe1.85O4, x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) and CoY- ferrites (CoFe2O4 + x Y2O3, x = 0 wt %, 1 wt %, 3 wt %, 5 wt %) were fabricated by co- precipitation method. The samples were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Vibrating Sample Magnetometery (VSM) and Impedance spectroscopy and Ferromagnetic Resonance. Phase analysis of NiY-, MgY- and NiZnY-ferrites from XRD patterns has shown cubic spinel single phase along with few traces of second phase identified as orthorhombic phase. This phase becomes more conspicuous for higher concentration of yttrium. The lattice constant as a function of yttrium contents changes non-linearly. The behavior of the lattice parameter was explained on the basis of differences in ionic radii of the constituent ions. Analysis of the XRD patterns of the CoZnY-ferrites confirms the formation of cubic spinel phase along with second phase of YFeO3. The lattice seems to expand to accommodate the increased number of Zn2+ ions of relatively larger ionic radii. The phase analysis of the XRD patterns of CoY-ferrites shows that all the samples are dual phase except the sample with x = 0 wt %. The lattice constant was found to decrease with yttrium contents. The lattice seems to compress by the presence of second phase due to difference in thermal expansion coefficients. X-ray density and physical density was found to increase whereas porosity was found to decrease with the increase of yttrium contents. The morphology of the samples shows non-homogeneous distribution of grains in all the samples except CoZnY-ferrites. The near uniform distribution of grain size was observed in CoZnY ferrites. FTIR spectra of NiY-, MgY- and NiZnY-ferrites observed at room temperature in the wave number range 370 – 1100 cm-1 exhibit splitting of the two fundamental absorption bands, thereby confirming the solid state reaction. FMR spectra of NiY- and MgY-ferrites were measured at room temperature at X-band (9.5 GHz). The nominal compositions MgY0.04Fe1.96O4 and NiY0.12Fe1.88O4 have small linewidth, ΔH = 269 Oe and 282 Oe respectively. Hence these ferrites have potential for high frequency applications. A systematic study of variations in resistivity with different concentration of yttrium has been carried out to optimize the resistivity. The room temperature resistivity shows an increasing trend in all series whereas it was decreased in case of Co-Zn-Y ferrites. The addition of Y3+ ions in place of Fe3+ ions reduce the degree of conduction by blocking Verwey’s hopping mechanism resulting in an increase of resistivity. The temperature dependent dc resistivity was found to decrease linearly with rise in temperature. The observed decrease in dc resistivity with temperature is normal behavior for semiconductors which follows the Arrhenius relation. It was observed that the samples having higher values of resistivity also possessed higher activation energy. The saturation magnetization was observed to decrease with yttrium contents which are due to redistribution of cations on the tetrahedral and octahedral sites. The coercivity was observed to increase with yttrium contents. The smaller grains may obstruct the domain wall movement. As a result, the values of initial permeability ( μ i′ ) decreased from 110 to 35, 27 to 6 and 185 to 87 at 1 MHz in NiY-, MgY- and NiZnY- ferrites respectively. The values of magnetic loss tangent decreased from 0.23 to 0.03, 0.04 to 0.007, 1.2 to 0.41 in NiY-, MgY- and NiZnY-ferrites respectively. This may be attributed to the increase in resistivity that reduces the eddy current loss. The frequency dependent behaviors of dielectric constant follow the Maxwell–Wagner’s interfacial polarization in accordance with Koops phenomenological theory. The introduction of yttrium ions decreases the dielectric constant and dielectric loss tangent (tan δ). The results obtained are of great interest for the development of modified spinel ferrites for various industrial applications.