PurposeIn his article titled, “The theory of the business,” Peter Drucker suggests that organizations need to frequently challenge the assumptions underlying their theory of the business (TOB) in order to assure that it remains viable. Accordingly, the purpose of this paper is to introduce the repertory grid technique (RGT) and illustrate how it can be used as a strategic planning tool to surface the assumptions underlying an organization's TOB.Design/methodology/approachThe RGT's merits as a planning tool are demonstrated through a case study involving the president of a Pittsburgh‐based financial services organization. Specifically, this case study describes how the RGT and the organization's competitors were used to surface the assumptions underlying the organization's TOB.FindingsThe paper describes how the assumptions surfaced from this case study can be used to challenge the organization's TOB. Furthermore, guidance is offered as to how the RGT can be integrated into an organization's strategic planning process and a list of resources that one can refer to for further information concerning the RGT is provided.Originality/valueWhile the RGT has been used by scholars as a research tool and various authors have suggested that it can be used to address various organizational challenges (e.g., implementing a new system), no one to date has demonstrated its value as a strategic planning tool. Consequently, the paper is a valuable resource for practitioners, as well as academics and consultants who provide strategic planning services to organizations.