INTRODUCTION The main aims of this Item are (i) to serve as an introduction to a series of Data Items@ in which detailed numerical data are given@ (ii) to provide guidance on simplified methods of calculating pressure losses@ (iii) to discuss some of the more general aspects of the estimation of pressure losses in duct flow systems@ (iv) to draw attention to the limitations that practical factors impose on the precision with which calculations can be made@ (v) to present conversion factors for some of the quantities that appear in pressure loss calculations. This Item covers a wide range of practical internal flow problems@ but it is restricted to the steady@ single-phase* flow of Newtonian fluids. See Section 9 for information on Items on non-Newtonian flow@ fans@ pumps@ noise in air-conditioning systems@ and fluid transients and their interaction with the pipeline. Although variations of flow and fluid properties occur at all cross sections of a real flow@ it is usual in practice to work in terms of averaged quantities. The equations given in this Item are based on the use of such quantities. * A glossary of terms is provided in Section 10.