Real-time monitoring of the temperature in supermarket refrigerators, testing air quality, detecting presence or position of a person indoors, monitoring the structural integrity of bridges, and measuring soil humidity for precision agriculture are only a few applications of wireless sensor networks. A wireless sensor network (WSN) is a system of spatially distributed autonomously powered embedded devices that are used to measure and collect one or more magnitudes, detect events, or both. Those devices are called wireless sensor nodes and can perform measurement or detection either independently or in cooperation with other sensor nodes of the network. WSN systems are used in applications that require distributed measurement points or mobility of the measurement points during operation. The subject of this thesis is the design of energy-efficient wireless sensor nodes and wireless sensor networks for the sub-GHz industrial scientific and medical (ISM) band, focusing on the hardware, firmware and communication protocol. Hence, the contributions span several different aspects of wireless sensor network design. A new robust wireless sensor node platform is presented; it was fully characterized and used to build a demonstration WSN that is in operation providing field results for more than two and a half years. The development, production and testing of the new wireless sensor node named RoSe (Round Sensor) is presented. Details are given on the choices of components, design priorities, production, and testing of a small prototype series. The node features a robust overmolded antenna-housing and two battery configurations. The parameters of the node are compared with those of an existing commercial sensor node that inspired the new development. During development of the low-level routines, a new technique was introduced that enables debugging and troubleshooting of an operating sensor node by using the supply current waveform measurement and a logic output. The second part of the thesis presents per-task charge consumption measurements of the RoSe node both in test conditions and while operating in the demo WSN. These measurements serve as a base for estimating the battery lifetime for a known application and communication protocol. The battery lifetimes of the nodes operating as a part of the demo WSN are reported, and by analyzing those results, battery lifetime estimations for different measurement intervals are calculated. Furthermore, a generalized overview of the ensemble of WSN parameters is given. Those parameters are then classified in groups and their interconnections are explained. Then, the network capacity limitations in terms of the number of sensor nodes and measurement intervals are derived based on radio frequency regulations and parameter interconnections. Recognizing that compliance with radio frequency regulations in Europe is one of the key constraints in single channel sub-GHz WSN development at present, we propose a new approach to regulatory compliance by introducing a firmware module for ensuring real-time regulatory compliance. The proposed firmware module was implemented and its memory and energy footprints were evaluated. Finally, we identified the need for a hardware module that will serve as an energy usage optimization block between the battery and the rest of the sensor node. The implementation of such a module requires both hardware and firmware development. To demonstrate the concept, a simplified hardware module was designed using a commercial off-the-shelf DC-DC converter and supercapacitors. The module was implemented and characterized in a new design iteration of the RoSe node and showed a possible battery autonomy extension of 19–28 %. Practical aspects of designing and building sensor nodes and a WSN are interwoven into the chapters of the thesis for completeness.