Cyber threat hunting is the practice of proactively searching for latent threats in a network. Engaging in threat hunting can be difficult due to the volume of network traffic, variety of adversary techniques, and constantly evolving vulnerabilities. To aid analysts in identifying techniques which may be co-occurring as part of a campaign, we present the Technique Inference Engine, a tool to infer tactics, techniques, and procedures (TTPs) which may be related to existing observations of adversarial behavior. We compile the largest (to our knowledge) available dataset of cyber threat intelligence (CTI) reports labeled with relevant TTPs. With the knowledge that techniques are chronically under-reported in CTI, we apply several implicit feedback recommender models to the data in order to predict additional techniques which may be part of a given campaign. We evaluate the results in the context of the cyber analyst's use case and apply t-SNE to visualize the model embeddings. We provide our code and a web interface.