This study attempts to identify, describe and quantify the problem of allegedly incompetent teachers in Irish voluntary secondary schools through the perceptions of their principals. Its central thesis is that the nature, extent and effects of teacher incompetence in Irish voluntary secondary schools are similar to those in the international experience. The thesis is argued by developing eight inter-related themes. 1. The nature of incompetence; 2. The existence and extent of the problem; 3. Incompetent teachers; 4. Attitude of the teacher union; 5. The principals' attitudes; 6. The principals' reactions; 7. Support and remediation; 8. Dismissal and induced exits. A postal questionnaire containing 46 questions and spaces for optional comments was administered to every voluntary secondary school principal in the Republic of Ireland. The response of 325 represents a return rate of 75%. Principals were asked to accept a given strict definition of an incompetent teacher and then to estimate how many of their teaching staff fitted that description. Then they filled in a type of 'census form' describing the characteristics of each incompetent teacher, noting their effects and examining the attitudes and reactions of management and detailing the outcomes of interventions. This produced 257 variables as well as written comments. The data were analysed using simple frequencies, descriptive statistics, cross-tabulations, chi-squares, ANOVA and T-tests. Principals assess teacher incompetence at six percent. Management reaction includes ineffective remediation attempts and accommodation by judicious timetabling. Principals expressed concern and felt that the issue needed to be addressed.