In parallel to a stand-alone microsphere resonator and a planar ring resonator on a wafer, the liquid core optical ring resonator (LCORR) is regarded as the third type of ring resonator that integrates microfluidics with state-of-the-art photonics. The LCORR employs a micro-sized glass capillary with a wall thickness of a few microns. The circular cross section of the capillary forms a ring resonator that supports the whispering gallery modes (WGMs), which has the evanescent field in the core, allowing for repetitive interaction with the analytes carried inside the capillary. Despite the small physical size of the LCORR and sub-nanoliter sensing volume, the effective interaction length can exceed 10 cm due to high Q-factor (106), significantly improving the LCORR detection limit. The LCORR is a versatile system that exhibits excellent fluid handling capability inherent to capillaries and permits non-invasive and quantitative measurement at any location along the capillary. Furthermore, the LCORR uses the refractive index change as a transduction signal, which enables label-free detection. Therefore, the LCORR is a promising technology platform for future sensitive, miniaturized, lab-on-a-chip type sensors. In this paper, we will introduce the concept of the LCORR and present the theoretical analysis and the experimental results related to the LCORR sensor development.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.