Internal auditing (IA) has become an indispensable control mechanism in both public and private organisations. Yet very few academic studies have been conducted on the effectiveness of IA. The current exploratory study aims to build a conceptual understanding of the effectiveness of IA in organisations. Towards this end it develops a scale to measure the effectiveness of IA and a model of its determinants. One hundred and eight Israeli organisations that employ IA participated in the study (a 37% response rate). Data on the effectiveness of IA were collected from the organisations’ general managers and data on the determinants from their internal auditors. The findings reveal good psychometric properties for the scale developed in this study. The correlation and regression analyses show support from top management to be the main determinant of IA effectiveness, with some effect also found for the organisational independence of IA. The effect of the predictors was consistent between the public and private sectors. The research model explained a large amount of variance of IA effectiveness. The findings are discussed in terms of how they can help guide and encourage the continuation of research on this issue.