Pig breeding is generally conducted among many herds, so EBV comparisons across populations are necessary. Genetic connectedness is required for reliable between-farm animal EBV comparisons. Five quantitative overall connectedness measures among populations have been proposed so far, coefficient of connectedness(γ*), coefficient of determination (CD) and overall indices of precision, connectedness rating, number of direct genetic links between subpopulations due to common sires and dams (GLt), and average genetic covariance (AGC) are reviewed and their properties are discussed in this paper. It is recommended to use AGC at present for measuring genetic connectedness between herds.