Air-kerma strength is the brachytherapy source strength quantity of interest in the United States and is realized using the wide-angle free-air chamber at the National Institute of Standards and Technology for low-energy, low-dose rate brachytherapy sources and through the use of the seven-distance apparatus at the University of Wisconsin Accredited Dosimetry Calibration Laboratory for high-energy, high-dose rate brachytherapy sources. The wide-angle free-air chamber is a primary standard, while the seven-distance apparatus is an interim standard. Both devices may be used to calibrate brachytherapy sources that, in turn, may be used to calibrate re-entrant well-type ionization chambers. This chapter describes the theory, design, and application of the wide-angle free-air chamber, seven-distance apparatus, and re-entrant well-type ionization chambers. The chapter concludes with a discussion about uncertainties.