Technetium has probably the most interesting and multifaceted chemistry amongst the metallic radionuclides in routine applications for radiopharmacy or molecular imaging. Being situated in the middle of the periodic table, the diversity of its possible oxidation states enables complexes with ligands covering any possible class of coordinating organic molecules and ions. However, the plethora of compounds resulting from this broad chemistry did not translate into a comparable number of new imaging agents. Reasons for this fact are discussed and practical requirements to which new potential radiopharmaceuticals have to comply with are outlined. Despite the chemical diversity of feasible complexes, the large majority of complexes concentrates on the three cores [99mTcV=O]3C, [99mTcV≡N]2C, and fac-[99mTcI(CO)3]C. Labelling strategies with these cores and classes of imaging agents are discussed and scrutinized. The more recent coordination chemistry of these cores is highlighted with a focus on new complexes, which are either fundamentally new or carry targeting functions for receptor binding with the known cores. Many of them have been subjected to in vitro and in vivo studies, important in the context of this chapter. It covers about the last ten years but references to earlier studies are given where necessary. Very few of these complexes proceeded to preclinical or even clinical studies. These are presented together with already market-introduced examples in Section 4. Throughout the chapter, the chemistry of rhenium plays a pivotal role for the one of technetium. This feature of rhenium is emphasized not only as a model for technetium using cold rhenium but also as an option for complementing imaging with 99mTc with therapy by radioactive 186/188Re, leading to a theranostic matched-pair situation as a perspective. As a scope for the future of fundamental technetium and rhenium chemistry, novel building blocks and strategies such as the integrated concept are introduced. It will corroborate the relevance of fundamental 99Tc chemistry for keeping 99mTc as a core imaging radionuclide at the forefront of nuclear medicine.