Positron emission tomography (PET) and single-photon emission tomography (SPET) are cross-sectional, quantitative functional imaging modalities in routine use in oncology for the initial staging of cancer, the assessment of patients with recurrent or residual disease and, more recently, for monitoring tumour response to therapy. Both PET and SPET can track tumour biological and metabolic changes caused by therapy or by disease progression, which usually precede the anatomical alterations conventionally detected by anatomical imaging methods. These highly sensitive functional imaging modalities have been used for the early assessment of subclinical tumour response, the evaluation of therapy after its completion and the detection of viable recurrent or relapsing tumour. Timely assessment of response to treatment using PET and SPET may result in modifications in treatment planning and individualisation of therapy and may have prognostic value for the long-term outcome. This review attempts to summarise the current data available on the expanding role of SPET and PET, using a variety of tracers, in monitoring tumour response to therapy in a wide range of malignancies, with emphasis on their clinical impact.