Optical probes, particularly the fluorescent varieties, enable researchers to observe cellular events in real-time and with great spatial resolution. Optical Probes in Biology explores the diverse capabilities of these powerful and versatile tools and presents various approaches used to design, develop, and implement them. The book examines the use of optical probes to detect and track numerous molecular processes in living cells, including GTPase and kinase activities, membrane lipids, voltage, metal ions, metabolic signals, RNA, and histone modifications. It critically reviews the different probe designs and delves into the strategies for developing new fluorescent protein varieties with enhanced capabilities. It also covers sophisticated imaging techniques and equipment, such as intensity and lifetime-based fluorescence microscopy methods, used to visualize and track optical probes.