Automatic image captioning, which involves describing the contents of an image, is a challenging problem with many applications in various research fields. One notable example is designing assistants for the visually impaired. Recently, there have been significant advances in image captioning methods owing to the breakthroughs in deep learning. This survey paper aims to provide a structured review of recent image captioning techniques, and their performance, focusing mainly on deep learning methods. We also review widely-used datasets and performance metrics, in addition to the discussions on open problems and unsolved challenges in image captioning.