This book deals with fields which possess different spins so, that there is an additional internal symmetry of such schemes. The main feature of theories with multi-spin is that transformations of the Lorentz and internal symmetry groups do not commute each other. So, parameters of the group are tensors but not scalars as in the more common-gauge theories. This kind of symmetry is also different from supersymmetry where group parameters are not scalars. The difference is that in this case the algebra of generators of the symmetry is closed without adding the generators of the Poincaré group. Consideration of schemes of fields with multi-spin possess some attractive features and avoid some difficulties but the price of this is the presence of the indefinite metric. It should be noted that some quantum theories of gravity also require the introduction of an indefinite metric. The localisation of parameters of the internal symmetry group leads to the gauge fields and field interactions. In Part II solutions are found for equations for particles with different spins in external classical and quantified electromagnetic fields, as well as the interaction of fields with multi-spin with external electromagnetic fields of different configurations.