One of the most interesting features in Solidity is the function modifiers. They are special control methods that are designated by a special keyword, modifier. Their role is to modify the behavior of defined functions, according to specific conditions. For example, they can automatically check a condition prior to executing the function. Consider the two following modifiers:

modifier an_ongoing_auction() {     
    require(now <= auction_end);
    _;    
}

modifier only_owner() {
    require(msg.sender == auction_owner);
    _;
}

The first checks whether the auction is still open, whereas the second restricts the authorization to executing a function to the contract's owner (auction_owner). The usage of the only_owner() modifier makes the contract ownable, which is a common smart contract pattern, thereby giving the contract owner specific privileges, similar to an admin in other systems. Generally, in modifiers, we check conditions by using the require, revert, or assert functions. The underscore in the modifier's body will be substituted by the code of the function being modified.