A number of constructions in English are hard to classify as clause or clause complex when they contain non-finite components. The key to identifying their status is to decide whether the non-finite component constitutes a non-finite clause. This paper uses insights drawn from Systemic Functional Grammar to suggest some criteria for non-finite clause identification. Consideration is given first to criteria from the ideational perspective (process, participant and relator) and second to criteria from the interpersonal. It is concluded that these criteria are helpful in a number of problem cases, but they are not a solution for all types of clauses with non-finite components, and more studies are needed to provide better generalization.