Classical galactosemia is caused by the genetic deficiency of galactose-1-phosphate-urydyl-transferase resulting in clinical symptoms development during the first weeks of life including jaundice, hypotonia, lethargy, emesis, hepatomegaly, among others. Currently, dietary restriction of galactose is considered the standard for classical galactosemia management. For several years, severe dietary galactose restriction was considered necessary, implying restriction not only of dairy products, but also fruits, vegetables, legumes, and viscera. Such management failed to improve or prevent the appearance of long-term complications, by contrast, such restrictive approach may lead to nutritional deficiencies development. Thus, the last consensus suggests guidelines that are more flexible. In addition, the lack of knowledge regarding the physiopathology of the disease, and the toxicity threshold of the metabolites accumulated, make even more difficult to propose novel and more effective therapeutic approaches. This review summarizes the current state of knowledge regarding classical galactosemia in terms of physiopathology, long-term complications, newborn screening and genetic variants and their implications on galactosemia treatment, summed to the challenges that researchers working on this disease must address in future studies including the analysis of galactose content in foods, galactose tolerance threshold and search for novel therapeutic targets.