Teaching "out-of-field" occurs when teachers teach a subject for which they are not qualified. The issues around this increasingly common practice are not widely researched and are under-theorised. A qualitative pilot study using teacher interviews in 3 rural schools examined meanings, support mechanisms and teacher identities associated with out-of-field teaching. A thematic analysis isolated factors influencing whether teachers self-assessed their practice and identities as out-of-field. The "boundary between fields" model was developed to emphasise support mechanisms, contextual factors and personal resources that influenced the nature of teachers' negotiation of subject boundaries and its impact on professional identity. These findings provide insight for policy makers, school leaders and teacher educators into the conditions required for such teaching to be considered learning opportunities.