Retinal　image　quality　assessment　(RIQA)　is　one　of　the　key　components　in　screening　for　diabetic　retinopathy　(DR).　As　one　of　the　most　serious　complications　of　diabetes,　DR　has　become　a　leading　cause　of　blindness　in　adults　globally.　DR　screening　is　essential　to　achieve　early　diagnosis　so　that　effective　treatment　could　be　provided　timely.　However,　the　collected　images　of　medically　unsatisfactory　quality　always　lead　to　failure　of　diagnosis　and　waste　of　ophthalmologists＇　precious　time.　Hence,　the　first　step　in　a　good　DR　screening　program　is　verifying　retinal　images　of　good　quality.　In　this　paper,　we　provide　a　systematic　review　on　automated　assessment　of　retinal　image　quality　for　DR　screening.　Scheme　and　parameters　for　RIQA　are　firstly　presented.　Next,　we　provide　detailed　understanding　of　the　existing　RIQA　techniques,　algorithms　and　methodologies,　including　brief　description　and　analysis　of　each　existing　state-of-art　approaches　and　comparison　between　such　methods.　Datasets　and　evaluation　metrics　are　also　illustrated.　Finally,　several　challenges　and　future　research　directions　are　summarized　and　discussed.