Noninvasive in vivo molecular-genetic imaging has developed over the past decade and involves nuclear (PET, gamma camera), magnetic resonance, and in vivo optical imaging systems. Although three different imaging strategies - "direct," "indirect" and "surrogate" - are being used, most current in vivo molecular imaging strategies are "indirect" and involve the coupling of a "reporter gene" with a complimentary "reporter probe." Imaging the level of probe accumulation provides indirect information related to the level of reporter gene expression. Reporter gene constructs are driven by upstream promoter/enhancer elements; reporter gene expression can be "constitutive" leading to continuous transcription and used to identify the site of transduction and to monitor the level and duration of gene (vector) activity. Alternatively, reporter gene expression can be "inducible" leading to controlled gene expression. Controlled gene expression can be tissue-specific and/or responsive to the level of endogenous promoters and transcription factors. Several examples of imaging endogenous biological processes in animals using reporter constructs, radiolabeled probes and PET imaging are reviewed, including: 1) imaging transcriptional regulation (e.g., p53-dependent gene expression), 2) imaging weak promoters (cis- vs. trans-reporter configurations), 3) imaging post-transcriptional regulation of gene expression, 4) imaging protein-protein interactions. The development of versatile and sensitive assays that do not require tissue sampling will be of considerable value for monitoring molecular-genetic and cellular processes in animal models of human disease, as well as for studies in human subjects in the future. Non-invasive imaging of molecular-genetic and cellular processes will compliment established ex vivo molecular-biological assays that require tissue sampling, and will provide a spatial as well as a temporal dimension to our understanding of various diseases.