Biomedical optics is an interdisciplinary field of combining principles of optics with a variety of technologies in biology, biophysics, biochemistry, medicine, computer science and bio-related engineering. This field has been gaining a great interest due to unique characteristics of light, for example, high spatial resolution, three-dimensional and real-time detection, noninvasive or minimally invasive procedure and simple manipulation. Such light-based research encompasses a wide range of basic study, inspection, diagnostics and therapy of biological cells and tissues. In order to keep pace with rapid advancement and to introduce current research trends, we select biomedical optics as a theme of this Special Issue of Biomedical Engineering Letters. This Special Issue includes five review articles and one original research paper covering various state-of-the-art technologies. The first paper entitled “The potential of naturally occurring lasing for biological and chemical sensors” by Choi and Kim introduces alternative biosensing methods of taking advantage of naturally occurring lasers, also known as random lasers [1]. In random lasers, resonances are selfformed due to multiple scattering, leading to light amplification and coherent light generation in the presence of amplifying media. In this case, lasing emission can be unprecedentedly sensitive to subtle nanoscale perturbations. The authors address that random lasers can be an alternative yet superior physical mechanism for biosensors, because the random laser biosensing platform is simple and the detection strategy is straightforward. The second paper entitled “Acoustic resolution photoacoustic microscopy” by Park and colleagues reviews the principle and system implementation of acoustic-resolution photoacoustic microscopy (AR-PAM) [2]. Note that, by combining information provided by light and acoustic properties, it is possible to realize high sensitivity and specificity, strong contrast of biological tissues, and relatively deep penetration beyond the depth limitation of traditional optical imaging. The authors also present several applications of AR-PAM to image biological tissues such as sentinel lymph nodes, lymphatic systems, bladders, gastrointestinal tracts, and whole body of small animals. Owing to its outstanding imaging capability, AR-PAM is anticipated to be applied for a variety of biomedical imaging researches. The third paper entitled “Recent functional near infrared spectroscopy based brain computer interface systems: Developments, applications and challenges” is described by Phillips V and Kim [3]. Portable and convenient functional near infrared spectroscopy (fNIRS) can provide an enormous Kyung Min Byun ( ) Kyung Hee University, Yongin, Republic of Korea Tel : +82-31-201-3842 / Fax : +82-31-202-1723 E-mail : kmbyun@khu.ac.kr