In this work, we review current knowledge on protein and molecular structures of lactoproteins, as well as the discovery and characterization of genetic polymorphism in the six major lactoproteins of cow (Bos taurus), goat (Capra hircus) and sheep (Ovis aries): α_s1, α_s2, β y κ caseins, and seroproteins α‑lactoalbumin and β‑lactoglobulin. Research is much more advanced in the bovine species, where evidence of genetic variability of all these proteins exists, the number of alleles ranging from four (α_s2 casein) to twelve (β‑lactoglobulin). Although more confusing, studies in goats also confirm the polymorphic status of the whole family of lactoproteins. In this species, however, attention has been drawn to the unusual complexity of (α_s1 casein polymorphism, which is under control of at least seven protein variants and twelve autosomal alleles. Moreover, these variants influence α_s1‑casein content in milk and thus exert a quantitative effect on the overall protein yield. As far as the ovine species is concerned, available information is still more controversial: it is nowadays evident the existence of genetic polymorphism in both α_s‑caseins, α‑lactoalbumin, and β‑lactoglobulin, whereas electrophoretic heterogeneity observed in case of β and κ caseins is due to distinct post‑translational events. In sheep, an additional source of non‑genetic polymorphism has been described for α_s‑caseins, which coexist in milk as two proteins of different lengths. The relevance of the tight linkage disequilibrium existing among the four casein genes is further on emphasized —the haplotype concept—. Finally, we revise studies that correlate certain casein variants and various physico‑chemical and technological properties of milk, which justify the interest of early allelic detection in breeding or industrial programs.