Optical nanosensors are advanced analytical tools that combine advantages of conventional sensor technology with flexibility in use of dissolved indicators. Introduced a decade ago, optical nanosensors attract increasingly more attention each year. In fact, the number of reports on the topic has doubled in the last 2-3 years. In this contribution, we aim to highlight recent developments of optical nanosensors and give an overview on strategies to design these nanosensors with different levels of complexity. Applications of this smart technology, advantages, challenges, and risks will be discussed.