Some of the best-established applications of molecular logic are found in optical sensing of chemical species and properties, particularly within cells and in tissue. These are based on YES and NOT logic and superpositions thereof. The last option is particularly valuable for fluorescence ratiometry which eliminates many problems encountered during cell studies. Measuring electrolytes in blood and air pressure on aerofoils are two of the most commercially successful applications, though others which sense marine toxins, nuclear waste components and chemical warfare agents are also coming though. Sensing can also be improved in different ways with AND, INHIBIT, TRANSFER, XOR logic and some superpositions thereof. Measuring concentrations of multifunctional molecules, apoptosis detection, tumour visualization and near-simultaneous monitoring of multiple species come under this umbrella. Some aspects of medical diagnostics, conventional therapy and photodynamic therapy can be improved in several ways as well. Various molecular logic tags allow a useful way of identifying small objects in populations, which is a new direction for developing applications. Another new direction is intracellular computation which benefits greatly from genetic engineering techniques. Being the last Chapter, it is also an opportunity to draw together all the threads to show that Boolean ideas permeate much (bio)chemistry.