This document provides an overview of object detection techniques including region-based and region-free methods. Region-based methods like R-CNN, Fast R-CNN, and Faster R-CNN first generate region proposals then extract features from those regions to classify and regress bounding boxes. Region-free methods like YOLO, YOLOv2, and SSD predict bounding boxes and classifications directly from the image in one pass. Both approaches are trained end-to-end using techniques like RoI pooling and anchor boxes to predict multiple detections. Recent work aims to improve speed and accuracy by generating detections sequentially or using soft NMS instead of hard thresholding.