Phosphorus (P) deficiency impairs productivity of ruminants and is the most expensive mineral used in supplements for cattle and buffalo under Brazilian grazing conditions. There is a need for the rational use of P, particularly under extensive grazing conditions, as this mineral resource is expensive and the world’s supplies are exhaustible. The diagnosis of P deficiency must start with careful and detailed history taking, followed by clinical examination of the herd and the use of ancillary tests. The latter are complementary to the diagnosis; they should be neither the first nor the most important steps when conducting a professional diagnosis. The indication of corrective or prophylactic measures necessarily involves the correct diagnosis of P deficiency (in herds), whether it be clinical or subclinical. This review discusses the main aspects related to P for cattle and buffaloes under grazing conditions in Brazil.