A centralizer for well casing (16) functions in an annular space between the casing (16) and a wellbore (20). It comprises a pair of axially spaced-apart and aligned collars (12) adapted to encircle the well casing and a plurality of spring bows (14) extending between and secured to the collars. Each spring bow (14, 280) has two ends and an outwardly convex curved mid-portion. In a first form of the invention starting force is reduced by providing spring bows having outwardly projecting contact angle reduction members (282) that engage the upper edge of the wellbore (20) as the casing (16) is moved into the wellbore (20) to reduce the effective contact angle of the spring bows (14, 280) with respect to the wellbore (20) and thereby reduce the force required to insert the centralizer (10) into the wellbore (20). In a second form of the invention the spring bows are provided with casing abutment members (283) extending inwardly towards the centralizer's longitudinal axis. The casing abutment members (283) contact the casing (16) as the spring bows (14, 280) are moved towards the casing (16) during insertion of the centralizer (10) into the wellbore (20) to increase the restoring force of the spring bows. The spring bows may have both contact angle reduction members (282) and casing abutment members (283) to give a centralizer having a desirable combination of low starting force with high restoring force.