Dendrimers and dendrons are classes of highly branched "tree-shaped" nanosized molecules. This chapter deals with dendrimers in connection with biology and medicine, and begins with a discussion on their properties. In a divergent approach of synthesis, the dendrimer is essentially synthesized beginning at the core and building outward, whereas in the convergent approach the dendrimer is built starting from the periphery and going toward the core. Polyamidoamine (PAMAM) dendrimers, are synthesized by Michael addition of methylacrylate to polyamines followed by amidation by treatment with an excess of 1,2-ethanediamine. The chapter highlights toxicity of dendrimers, and then goes on to discuss the use of dendrimers in diagnostics and dendrimer-based reagents in imaging. Other topics that are discussed include anti-inflammatory activities of dendrimers, DNA transfection, and small interfering RNA (siRNA).