Our laboratory utilizes protein engineering to develop antibodies for targeted delivery in vivo for applications in oncology - diagnostic or therapeutic. In particular, we focus on the development of immunoPET tracers based on the engineered antibody fragment platforms we have also developed, including minibodies and diabodies. Furthermore, use of phage display libraries provides a source of novel, fully human antibodies that can ultimately be used in the clinic. Engineered antibodies that have been conjugated and radiolabeled with positron-emitting radionuclides, including F-18, Cu-64, Zr-89 and I-124, can be applied to non-invasive imaging of cell-surface phenotype, and can provide novel ways to assess tumor biology and response to therapy. More recently we have extended immunoPET to imaging immune cell subsets (such as CD8 cytotoxic T cells), providing a powerful approach for profiling immune responses which are central not only to emerging cancer immunotherapies, but in many other diseases as well.