Ultrasonic sensors are common sensors to use in proximity sensing and distance measurement applications. With the advent of Internet of Things (IoT), many new application possibilities have opened, especially concerning IoT devices, that form integral parts of larger and more complex solutions, such as smart cities or smart control systems. Because of that, there are many sensors specifically tailored to a narrow range of applications. The sensors vary widely in terms of maximum distance, accuracy, robustness, and price. We discuss the basic theory and operating principles of ultrasonic sensors and compare them to other alternative solutions for certain specific applications. Several different applications, with a focus on smart cities, are discussed. Further, we show that ultrasonic sensors can be used on both low-end microcontroller-based devices as well as powerful single board computers, according to target application requirements. A general IoT-based smart city system structure is briefly discussed.