This paper reviews work carried out at the National Engineering Laboratory, UK., related to pressure drop flow patterns and phase distribution on the shell-side of segmentally baffled shell-and-tube heat exchangers. The experimental work reported was carried out using air/water mixtures in model exchangers of rectangular cross section with tube nests containing approximately 40 tubes. Data were obtained on crossflow pressure drop and on the pressure drop attributable to the windows. In certain configurations the void fraction and flow pattern maps were obtained. The geometric conditions examined related to configurations appropriate to operation as condensers and boilers. Correlations for pressure drop and void fraction were developed and flow pattern maps obtained.