Continuous development of highly efficient and compact cooling elements are driven by extensive developments of Micro-Electro-Mechanical Systems (MEMS) devices such as microprocessors or micro high-powered laser systems. As microprocessors continue to head in the direction of miniaturization, it was quickly realized that the major obstacle to overcome was "heat". This justified the need for the development of a new generation of liquid cooling technologies that involves phase-change processes. However, significant differences in two-phase transport phenomena have been reported in the microscale sized channels as compared to conventional macroscale channels. The classification of macroscale, mesoscale and microscale channels with respect to two-phase processes is still an open question. This research project focuses on investigating the macro-to-microscale transition during flow boiling in small scale channels of various size and refrigerants to investigate the effects of channel confinement on flow boiling heat transfer, two-phase flow patterns, pressure drop, critical heat flux and liquid film stratification in a single circular horizontal channel. In this project, we systematically investigated the trends observed in the heat transfer data, pressure drop, critical heat flux and film stratifications for wide range of conditions as a function of flow pattern, creating a uniquely comprehensive experimental database for R134a (high pressure), R236fa (medium pressure) and R245fa (low pressure) in the 1.03, 2.20 and 3.04 mm channels. From a heat transfer viewpoint, the prediction of the heat transfer requires first the prediction of the two-phase flow regimes in these small scale channels. So, in that respect an improved flow pattern map has been proposed and by determining the flow patterns transitions existing under different conditions, the effects of the flow regimes and channel dimensions on flow boiling heat transfer is thus explained. A flow pattern based heat transfer prediction model, i.e. the three-zone model showed good agreement in the prediction of heat transfer coefficients corresponding to the isolated bubble (IB) and coalescing bubble (CB) flows. As for the two-phase pressure drops, various statistical comparisons have been made by comparing the pressure drop database with various macroscale and microscale prediction methods, finding that three macroscale and two microscale methods predicted the pressure drop database with good accuracy. From this research, a distinctive peak in the CHF has been identified when comparing the experimental CHF database for 0.50, 0.80, 1.03, 2.20 and 3.04 mm channels (the first two from previous study). A new CHF prediction method was developed that is applicable for the CHF prediction of single, square multi-microchannels and split flow multi-channels. Finally, the processing of the high speed flow visualization images provided the interpretation of the lower threshold of the macro-microscale transition. As an approximate rule, the lower threshold of macro-scale flow is Co= 0.3-0.4 while the upper threshold of the symmetric microscale flow is Co≈1.