Background: Ever since a few decades ago, application of immuno-PET in oncology has become an area of growing interest and importance. The combination of the high specificity and selectivity of antibodies with the excellent sensitivity of positron emission tomography (PET) predicts both high accuracy in cancer diagnosis and monitoring of tumor development following therapy. The aim of this chapter was to provide a summary of the basic knowledge on immuno-PET and some representative (mainly clinical) applications of immuno-PET in oncology. We then discussed the future evolution and developments needed for immuno-PET to contribute to the evolution of the clinical management of cancer patients. Methods: To identify available studies on immuno-PET approaches primarily in the clinic, we performed a PubMed search. The search terms “Immuno-PET,” “Cancer,” “Antibodies,” and “Clinical” were used in combination with the most commonly used PET radionuclides:64Cu,68Ga,86Y,89Zr, and124I. We focused on the most representative published studies and included relevant previously published works to provide context. In this regard, our search was limited to articles published in English. We point out that dissertation reports, theses, refereed conference papers, journals, and books were excluded. Results: Immuno-PET is a molecular imaging modality that combines the high specificity and high affinity of antibodies with the exceptional sensitivity of PET. This approach seems very attractive and should be a promising powerful diagnostic tool but remains difficult to implement. Indeed, radiolabeled molecules for immuno-PET are difficult to develop because of the multiple criteria to be met in terms of specificity, efficacy, and safety. During the last two decades, the preclinical development of new radiotracers for immuno-PET has been accelerating due to the increasing availability of suitable radioisotopes and the increase in the number of PET scans but also due to the increase in knowledge about potential cancer targets. However, clinical translation remains very limited, and further development and improvement will be necessary for immuno-PET to take its place in the arsenal of cancer diagnosis and theranostics. Conclusions: Immuno-PET imaging represents a powerful tool to detect primary tumors as well as distant metastases and provides phenotypic information that could guide therapeutic decisions and assist treatment monitoring. However, further significant improvements are required both to overcome the unfavorable pharmacokinetic parameters of monoclonal antibodies and to improve image analysis methods by integrating radiomics, for example.