Government and IT companies are investing and planning to adopt smart city concept in their cities. With the concept of smart city, multiple technologies to improve health, education, transportation, energy comes which needs millions of sensors to be deployed and a huge amount of data is generated by these nodes on a daily basis. From here comes the buzz word " Big Data ". A study by Mckinsey Global estimates that 43 trillion GB of data will be generated by 2020. In one second 22810 GB of internet traffic exists, 2325044 emails are sent, 7474 Tweets are posted, 1229 Instagram photos are uploaded [1]. Data is likely to grow drastically in the upcoming years. A large number of applications are equipped with sensors that measures multiple variable and generate a bulk amount of data. In this paper we are going to review the concepts of big data and how it supports smart cities in implementing smart technologies and making smart decision. This paper will also reveal that though various techniques are available but there are still some challenges to be overcome to achieve better utilization of big data.