A considerable number of transition geometries between ducts of varying cross-sectional area and/or shape are studied. In all cases, the analysis is based on straight-line generators along the transitions. Two area distributors are considered: firstly, area normal to the end duct centre lines; and secondly, area perpendicular to the transition centre line, in transitions with the inlet and outlet duct centre line offset. For the first case, the area variation is of a quadratic form along the transitions, however, for the second case, the variation is more complicated. A new design of some particular transitions, using curved walls to provide constant cross-sectional area along such transitions, is discussed. Experimental results are presented for the pressure losses in transition between square and rectangular ducts of the same cross-sectional area. These transitions include divergent-convergent, constant-area and sudden transitions. Effects of direction of flow, inlet conditions and surface roughness are examined. Sudden transitions have the highest pressure losses; however, the lowest pressure losses are obtained in the constant-area transitions. In all types of the transitions tested the pressure loss is a function of the aspect ratio at the rectangular end; where it is higher at lower aspect ratios. Numerical methods are introduced to solve the two-dimensional boundary-layer equations. An attempt is made to use this technique in predicting the performance of such transition ducts.