The crystallization behavior of random ethylene-1-alkene copolymers is reviewed. Major attention is focused on fractions and samples that have well-defined and narrow molecular mass and composition distributions. Emphasis is given to the analysis of thermodynamic properties, the basic elements of phase structure, the purity of the crystalline phase and the supermolecular structure. The molar mass, co-unit content and chemical type are treated as independent variables in the analysis of a large amount of experimental data. As is expected from theoretical considerations, the thermodynamic properties are very sensitive to the sequence distribution of the co-units in the chain. It is found that, except for directly bonded methyl groups, the crystallite structure remains pure, irrespective of the chemical nature of the co-unit. Thermodynamic properties and the major elements of phase structure, with the exception of the interfacial structure, are independent of the nature of the side group for groups larger than methyl. The analyses of a large amount of experimental data makes it clear that the molar mass and co-unit content need to be treated as independent variables.