This paper presents a summary of a 4-year investigation into the effect of flow obstacles on critical heat flux (CHF). The investigation was performed using a vertical 6.92 mm tube, cooled with R-134a. The tests covered a pressure range from 0.96 to 2.39 MPa, a mass flux range from 500 to 3000 kg m(-2) s(-1), and an outlet (critical) quality range from -0.05 to +0.95. The following flow obstacle effects on CHF were investigated: (a) the degree of flow blockage (blockage ratios of 12%, 24% and 37%); (b) the flow obstruction shape (cylinder, bar, plate, segment- and sector-shaped obstacles, rings, and twisted plate); (c) the leading and trailing edge shape (abrupt, knife shape, rounded edges); (d) the axial distance separating the flow obstacles, and the distance between the last obstacle and the downstream end of the heated length; (e) the number of obstacles in series; and (f) the location of obstacles within a cross-section. The test results showed that the presence of flow obstacles generally increases the CHF downstream of the obstacle, although the magnitude of the CHF increase depends on the shape and size of the flow obstacles, the number of obstacles per axial plane, the shape of the leading edge, and their circumferential location, as well as the flow conditions (critical quality, mass flux and pressure). An improved method for predicting the CHF increase due to the presence of flow obstacles was developed. This semi-analytical method correctly represents the observed parametric and asymptotic trends, including the impact of flow and quality. This method is considered to be a significant improvement over the existing CHF enhancement prediction methods.