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This chapter aims to provide a review of the basics of 21 cm interferometricobservations and its methodologies. A summary of the main concepts of radiointerferometry and their connection with the 21 cm observables - power spectraand images - is presented. I then provide a review of interferometriccalibration and its interplay with foreground separation, including the currentopen challenges in calibration of 21 cm observations. Finally, a review of 21cm instrument designs in the light of calibration choices and observingstrategies follows.

21 cm observations: calibration, strategies, observables