This study presents a comprehensive review on single and two-phase pressure drop characteristics and flow boiling instabilities in microchannels to outline the discrepancies in the literature. The effect of mass flux, heat flux, experimental conditions, channel geometrical parameters (hydraulic diameter, aspect ratio etc.) was surveyed critically over a broad range of literature studies including the past and recent researches. Additionally, conventional and micro-scale pressure drop correlations are discussed. This study showed that the continuum theory is applicable for single-phase pressure drop applications with some considerations. Also, the two-phase pressure drop in micro-scale was found to have similar characteristics with conventional-scale channels. On the other hand, flow boiling instability is one of the important issues in microchannel heat exchangers. Flow boiling instabilities affect the system performance and may cause system failures. Therefore, unstable flow boiling conditions were identified in this paper for a reliable design of micro heat exchangers.