HST observations of Cepheids in 37 hosts of 42 “ideal” SNe Ia and NGC 4258, collected over 20 yr with four cameras and >1000 orbits of HST time. In most cases, 60 to 90 day campaigns in F555W and F814W or in F350LP were used to identify Cepheids from their light curves with occasional observations years later to identify longer-period Cepheids. NIR follow-up observations in F160W are used to reduce the effects of host-galaxy extinction, sensitivity to metallicity, and breaks in the P–L relation. Data sources: (1) HST Key Project, Freedman et al. (2001); (2) HST SN Ia Luminosity Calibration Program, Sandage et al. (2006); (3) Riess et al. (2005); (4) Macri et al. (2006); (5) Mager et al. (2013); (6) Yuan et al. (2022).