A brief history of the many investigations of the pressure/temperature phase diagrams of elemental carbon is given. The strengths and weaknesses of the various proposed diagrams are discussed, and the areas where additional or better experimental data are needed are pointed out. Many questions still remain about linear solid carbons, about the low-pressure/high-temperature solid/liquid/vapor triple point; about the melting line of diamond; about the existence of a high-pressure metallic phase; and about the existence of two liquid phases, one insulator and the other metallic.