The 2002 edition of the Masonry Standards Joint Committee (MSJC, 2002) design provisions for masonry structures includes a chapter on the design of post-tensioned masonry (PTM) walls with either bonded or unbonded tendons. Recent research indicates that the MSJC formula for moment capacity of PTM sections can be inaccurate if unbonded tendons are used. In addition, the MSJC formulas have not been verified for slender walls. An experimental program is underway to investigate the resistance of slender PTM walls to transverse loading. Twelve 4.27-m (14-ft) tall walls with 810 × 100 mm (32 × 4 in.) cross-sections are being tested under monotonically increasing transverse loads. Six walls were made using clay brick, and the other six using concrete block. Some of the walls were post-tensioned using threaded steel rods that are not restrained from lateral deflection relative to the masonry, while the other six feature restrained tendons. The experimental investigation is outlined.