This　paper　mainly　presents　comprehensive　review　on　the　research　regarding　adiabatic　and　diabatic　gas–liquid　two-phase　flow　patterns,　bubble　growth,　flow　pattern　transitions　and　flow　pattern　maps　in　microchannels　over　the　past　15　years.　First,　criteria　for　distinction　of　macro-　and　micro-channels　are　discussed.　Then,　fundamentals　of　gas　liquid　two-phase　flow　patterns,　flow　pattern　maps　and　techniques　for　two　phase　flow　visualization　and　sensing　are　presented.　Next,　experimental　studied　of　adiabatic　and　diabatic　two　phase　flow　patterns,　bubble　behaviour,　flow　pattern　transitions　and　flow　pattern　maps　in　microchannels　with　plain　and　enhanced　structures　are　reviewed.　Finally,　applications　of　flow　patterns　and　flow　pattern　maps　are　discussed.　Flow　pattern　based　mechanistic　heat　transfer　prediction　methods　are　focused　on　and　studies　on　unstable　and　transient　two　phase　flow　patterns　and　heat　transfer　in　microscale　channels　are　addressed.　According　to　the　review　and　analysis,　recommendations　on　the　future　research　needs　have　been　given.　Systematic　and　accurate　experimental　data　on　flow　patterns,　bubble　growth,　flow　pattern　transitions　are　still　needed.　In　particular,　there　are　lacks　general　flow　pattern　transition　criteria.　Therefore,　effort　should　be　made　to　develop　generalized　flow　pattern　transition　criteria　based　on　well　documented　experimental　observation　and　data.　Furthermore,　studies　of　mechanistic　and　theoretical　models　for　flow　patterns,　flow　pattern　transitions　bubble　growth　in　microchannels　should　be　further　conducted.　As　an　important　topic,　unstable　and　transient　gas　liquid　two　phase　flow　patterns　and　heat　transfer　in　microchannels　should　be　systematically　investigated　as　well　in　order　to　understand　the　flow　pattern　transition　mechanisms　in　microchannels　with　plain　and　enhanced　structures.　©　2023　The　Author(s)