Unattended sensors can be divided into several classes based on their size and capabilities. Microsensors are small scale devices with low power requirements, processing, communication and storage capacity. Midsize sensor nodes have a higher power requirement but they have greater communication, storage and processing capabilities. There are a large number of diverse applications, where both microsensors and midsize sensors can be used successfully. In this chapter, the authors describe the most common architecture and applications for microsensors and for midsize sensors. The authors then focus on marine based applications of these sensors.