The innate immune system uses multiple strategies to detect viral infections. Because all viruses rely on host cells for their synthesis and propagation, the molecular features used to detect viral infections must be unique to viruses and absent from host cells. Research in the past decade has advanced our understanding of various cell-intrinsic and cell-extrinsic modes of virus recognition. This review examines the innate recognition from the point of view of virus invasion and replication strategies, and places innate sensors in the context of detecting viral genome, replication intermediate, transcriptional by-product, and other viral invasion strategies. On the basis of other unique features common to viral infections, undiscovered areas of virus detection are discussed.