We are pleased to present this focus issue on optical molecular imaging, which has seen new developments in novel optical imaging techniques and functional optical probes for studying molecular events in living organisms. This focus issue covers optical biosensor development, probe targeting, multispectral tomography, and new imaging and signal processing techniques that improve the study of molecular and cellular events in living cells and small animals. The ultimate goal is to use optical imaging techniques to monitor genetic expression and protein function during cellular processes and elucidate biological phenomena, leading to earlier detection of diseases, more effective evaluation of treatment effects, and new approaches to speed drug discovery and development. This issue includes two review articles and nine original papers.