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The structure and solidification of CoCrCu-X MPEAs with X = Fe, Mn, Ni, Ti, V, FeMn, FeNi, FeTi, FeV, MnNi, MnTi, MnV, NiTi, NiV, and TiV were studied via arc-melting and electromagnetic levitation melting. The ternary mixture of CoCrCu was found to form no single phase liquid, however by systematically introducing the remaining 3d transition metals, it was found that Ni and Ti promote single phase liquid formation, eventually leading to dendritic microstructures as opposed to the highly phase separated microstructures found in ten of the alloys. The thermodynamics of the liquid phase separation in these alloys is largely dictated by the positive mixing enthalpy contributions of Cu in these systems. Of the six dendritically solidifying alloys, CoCrCuNi, CoCrCuFeNi, and CoCrCuMnNi solidified with a face-centered cubic (FCC) crystal structure for both the dendritic and interdendritic phases while CoCrCuTi and CoCrCuTiV solidified with FCC and body-centered cubic (BCC) phases. In contrast CoCrCuMnTi solidified with a hexagonal closed packed Laves C14 dendritic phase and FCC interdendritic matrix. The three FCC alloys were then prepared via powder metallurgy and processed via spark plasma sintering (SPS) to compare microstructure, crystal structure, and mechanical properties with the solidification processed alloys. It was found that the powder metallurgical processing and SPS led to a doubling of the hardness in these alloys due to the nanocrystallinity of the powder being preserved. Liquid phase separation was further investigated by neutron imaging techniques. It was found that the technique not only allows for the direct observations of molten metals, but also shows mixing and de-mixing in the liquid for these alloys. The neutron imaging technique was applied to the CoCrCuNi high entropy alloy to study the remixing of the de-mixed liquid with the addition of Ni to CoCrCu. The CoCrCuMnTi alloys were systematically studied by varying the amount of Mn in order to find the critical Mn concentration for the formation of the Laves C14 phase in these alloys. The particular composition of Co22Cr18Cu20Mn16Ti24 has high hardness of 996.6 HV 0.01 for the dendritic phase, while the hardness of the interdendritic phase is 457.3 HV 0.01. In this alloy, there is also a small dispersed Ti-rich phase. The effects of varied cooling rates on this alloy were studied, and it was found that higher cooling rates led to the suppression of the tertiary Ti-rich phase.