The oligometastatic paradigm implies that patients who develop a small number of metastatic lesions might achieve long-term survival if all these lesions are ablated with surgery or stereotactic radiotherapy. Clinical data indicate that the number of patients with oligometastatic disease receiving aggressive treatment is increasing rapidly. We examine the key evidence supporting or refuting the existence of an oligometastatic state. Numerous single-arm studies suggest that long-term survival is 'better-than-expected' after ablative treatment. However, the few studies with adequate controls raise the possibility that this long-term survival might not be due to the treatments themselves, but rather to the selection of patients based on favourable inclusion criteria. Furthermore, ablative treatments carry a risk of harming healthy tissue, yet the risk–benefit ratio cannot be quantified if the benefits are unmeasured. If the strategy of treating oligometastases is to gain widespread acceptance as routine clinical practice, there should be stronger evidence supporting its efficacy.