# The Cosmos Hub Testnet
The Cosmos Network (opens new window) is an expanding ecosystem of connected blockchains. Many blockchains in the ecosystem are built with Tendermint Core (opens new window) and Cosmos SDK (opens new window). These blockchains are connected with the Inter-Blockchain Communication Protocol (IBC) (opens new window).
One of these blockchains is the Cosmos Hub (opens new window). Mainnet of the Cosmos Hub has been online since early 2019. You can learn how to join the network as a validator, see Run a Validator on the Cosmos Hub Mainnet (opens new window).
# What is a testnet?
A testnet is a network launched for developers and validators to test the functionality of the blockchain before the main network is launched. The state of a testnet can be reset at any time.
Now that you know what a testnet is, it's time to get started. See About the Testnet Tutorial.