Title: Parallel $Q$-Learning: Scaling Off-policy Reinforcement Learning. Abstract: Reinforcement learning algorithms typically require tons of training data, resulting in long training time, especially on challenging tasks. With the recent advance in GPU-based simulation, such as Isaac Gym, data collection speed has been improved thousands of times on a commodity GPU. Most prior works have been using on-policy methods such as PPO to train policies in Isaac Gym due to its simpleness and effectiveness in scaling up. Off-policy methods are usually more sample-efficient but more challenging to be scaled up, resulting in a much longer wall-clock training time in practice. In this work, we presented a novel parallel $Q$-learning framework that not only gains better sample efficiency but also reduces the training wall-clock time compared to PPO. Different from prior works on distributed off-policy learning, such as Apex, our framework is designed specifically for massively parallel GPU-based simulation and optimized to work on a single workstation. We demonstrate the capability of scaling up $Q$ learning methods to tens of thousands of parallel environments. We also investigate various factors that can affect the policy learning training speed, including the number of parallel environments, exploration schemes, batch size, GPU models, etc.