Spatial localization (co-location) of nodes in wireless sensor networks (WSNs) is an active area of research, with many applications in sensing from distributed systems such as micro-aerial vehicles, smart dust sensors, and mobile robotics. This paper provides a comprehensive review and comparison of recent implementations (commercial and academic) of physical measurement techniques used in sensor localization, and of the localization algorithms that apply these measurement techniques. Physical methods for measuring distances and angles between WSN nodes are reviewed, followed by a comprehensive comparison of localization accuracy, applicable ranges, node dimensions, and power consumption of the different implementations. A summary of advantages and disadvantages of each measurement technique is provided along with a comparison of co-localization methods in WSNs across multiple algorithms and distance ranges. A discussion of possible improvements to accuracy, range, and power consumption of selected self-localization methods is included in the concluding discussion. Although the preferred implementation depends on the application, required accuracy, and range, passive optical triangulation is reported as the most energy efficient localization method for low-cost/low-power miniature sensor nodes. It is capable of providing micron-level resolution, however the applicable range (inter-node distance) is limited to single centimeters.1530-437X (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2015.2450742, IEEE Sensors Journal SUBMITTED TO THE IEEE SENSORS JOURNAL 2 on 2.4 GHz radios are: Nordic Semiconductor nRF24L01 transceiver [16] with consumed power from 345 µW to 2.4 mW depending on the refresh rate of the location task, ZigBee Texas Instruments SoC CC2430 radio transceiver [12] with 10 mW to 40 mW consumption depending on implementation, RF Telosb transceivers [17] with consumption from 35 mW to 41 mW depending on the activity profile, and finally on custom radios for IEEE 802.15.4 build from digital ASIC standard cells technology [18] with power consumption of 15.6 mW at 0 dB output power. The communication system is used by a sensor node to perform its message-based self-localization task as well as for broadcasting the basic data collected by the sensor. Communication systems based on ultrasound waves have not been attempted due to the directional dependence of acoustic signals and their dependence on air temperature and humidity. Optical communication between sensors in WSN has been implemented recently using low-power laser diode emitters and detectors [19], modulating Corner-Cube Reflectors (CCR) [20], MEMS CCR arrays [21], MEMS steering mirors (gold coated, refl...