The magnetic flux distribution on the end faces of superconducting discs at 4.2°K has been observed by a Bitter pattern technique and compared with the metallurgical structure of the specimens. Second phase regions of CaPb3in two-phase lead-calcium alloys, compositional inhomogeneity in single-phase lead-indium alloys, and grain boundaries and surface scratches in V3Si are some of the features that give rise to flux pinning. The results suggest that the effectiveness of any pinning centre depends on its size in relation to the coherence length of the superconducting matrix and on the gradient of the Ginzberg-Landau parameter around the pinning centre.