Given that all sensor networks stream some forms of time varying measurements, it is not enough to guarantee confidentiality and authentication; we also must ensure each message is fresh. Informally, data freshness implies that the data is recent, and it ensures that no adversary replayed old messages. We identify two types of freshness: weak freshness, which provides partial message ordering, but carries no delay information, and strong freshness, which provides a total order on a request-response pair, and allows for delay estimation. Weak freshness is required by sensor measurements, while strong freshness is useful for time synchronization within the network.