This Field Book provides a simple guide to the control of volume to be removed when marking a thinning. There are three sections. The first section describes the Yield Class system and the assessment of yield class in a stand. The second section covers thinning practice, that is, the type, intensity and cycle of thinning, how to calculate the thinning yield, the timing of thinning, and how the thinning is controlled. The final section describes the field procedures for estimation of top height, basal area and volume marked, and how to calculate mean diameter. A checklist of the office and field procedures to be followed when marking a thinning is printed on page 56 inside the back cover. This publication is no longer available in hard copy.