Diagnosing cystic fibrosis (CF) in adulthood is not a rare occurrence for CF centers despite the popular belief that the diagnosis is achieved almost universally in childhood by means of newborn screening or early clinical presentation. The purpose of this review article is to highlight specific considerations of adult diagnosis of CF. Obtaining a diagnosis of CF at any age is exceptionally important to ensure optimal treatment, monitoring, and support. In the new era of more personalized treatment with the advent of transformative therapies targeting the underlying protein defect, accurate diagnosis is of increasing importance. This review highlights the diagnostic algorithm leading to a new diagnosis of CF in adults. The diagnosis is usually confirmed in the presence of a compatible clinical presentation, evidence of cystic fibrosis transmembrane conductance regulator (CFTR) protein dysfunction, and/or identification of variants in the CFTR gene believed to alter protein function. Achieving the diagnosis, however, is not always straightforward as CFTR protein function exists on a continuum with different organs displaying varying sensitivity to diminution in function. We highlight the current knowledge regarding the epidemiology of CF diagnosed in adults and outline the various clinical presentations, including pulmonary and extrapulmonary, which are more common in this population. We expand on the stepwise testing procedures that lead to diagnosis, paying particular attention to additional levels of testing which may be required to achieve an accurate diagnosis. There continues to be an important need for both pulmonary and other specialists to be aware of the potential for later presentation of CF, as the improvements in treatment over decades have had large positive impacts on prognosis for people with this condition.