Considerable advancements in the technology of wireless sensor networks (WSNs) are taking place due to smaller, and more efficient electronic devices which are capable of increased processing power, and communication capabilities. In addition, the cost of such devices is constantly decreasing, which makes it possible to employ large quantities of networked sensors in numerous commercial, environmental, military and health care applications. A lot of these applications involve lining up the sensor nodes in a linear structure giving rise to a new class of WSNs, which is defined in this work as Linear Sensor Networks (LSNs). This paper identifies some of the applications that might use such networks and offers a classification of the different types of LSNs from a topological and hierarchical points of view. Finally, a motivation for designing specialized protocols that take advantage of the linearity of such networks in order to increase reliability, efficiency, energy savings, and network lifetime is offered along with the new research issues, challenges and opportunities that exist in this field.