How can we make blockchains secure at scale? We suggest a data availability and validity scheme that make sharding efficient in terms of the number of validators and validating resources. We first describe the Polkadot data availability and validity scheme and consider its applicability to other sharded systems (e.g. ETH2.0). In Polkadot we tie an erasure coding data availability scheme with consensus, where we can not finalise an unavailable block. Moreover, reports of unavailability or invalidity trigger extra checks. The aim is that, with high probability, we do not finalise an unavailable or invalid block provided that there are enough honest actors to report. The key advantage of this scheme is that we need fewer validating actors per shard and in turn, less total computational, and especially, networking resources. This softens the trade-off between scalability and security.