Object detection, the task of localising and classifying objects in a scene, one of the most popular tasks in Computer Vision, has a main drawback: a large annotated dataset is necessary to train the model. Indeed, annotating a dataset is expensive, and the free available datasets are not enough, as they do not contain all the classes we are interested in. Thus, the goal of the tutorial is to introduce the main techniques to train a good object detector utilising the minimum amount of annotated data.