A modular computing system for a data center, comprising: one or more data center modules comprising one or more rack-mounted computer systems; one or more electrical modules coupled to at least one of the one or more data center modules and configured to provide electrical power to at least one of the computer systems in the one or more data center modules; and one or more air handling modules (104, 108) coupled to at least one of the data center modules, wherein at least one of the air handling modules comprises at least one fan, wherein the at least one air handling module is configured to provide air to at least one computer system in at least one of the data center modules, wherein at least one of the one or more air handling modules is mounted on top of one or more of the data center modules at an interface, wherein at least one of the one or more air handling modules comprises one or more supply air openings and at least one of the data center modules comprises one or more supply air openings, wherein at least one supply air opening of the at least one air handling module aligns with at least one supply air opening of the at least one data center module when the air handling module is coupled to the data center module, such that supply air can flow from the air handling module to the data center module via the aligned supply air openings, wherein at least one of the one or more air handling modules comprises one or more return air openings and at least one of the data center modules comprises one or more return air openings, wherein at least one return air opening of the at least one air handling module aligns with at least one return air opening of the at least one data center module when the air handling module is coupled to the data center module, such that return air can flow from the data center module to the air handling module via the aligned return air openings, wherein the interface is between the at least one of the one or more air handling modules and the one or more data center modules when the air handling module is coupled to the data center module, and wherein the one or more supply air openings and the one or more return air openings are located at the interface. one or more electrical modules coupled to at least one of the one or more data center modules and configured to provide electrical power to at least one of the computer systems in the one or more data center modules; and one or more air handling modules (104, 108) coupled to at least one of the data center modules, wherein at least one of the air handling modules comprises at least one fan, wherein the at least one air handling module is configured to provide air to at least one computer system in at least one of the data center modules, wherein at least one of the one or more air handling modules is mounted on top of one or more of the data center modules at an interface, wherein at least one of the one or more air handling modules comprises one or more supply air openings and at least one of the data center modules comprises one or more supply air openings, wherein at least one supply air opening of the at least one air handling module aligns with at least one supply air opening of the at least one data center module when the air handling module is coupled to the data center module, such that supply air can flow from the air handling module to the data center module via the aligned supply air openings, wherein at least one of the one or more air handling modules comprises one or more return air openings and at least one of the data center modules comprises one or more return air openings, wherein at least one return air opening of the at least one air handling module aligns with at least one return air opening of the at least one data center module when the air handling module is coupled to the data center module, such that return air can flow from the data center module to the air handling module via the aligned return air openings, wherein the interface is between the at least one of the one or more air handling modules and the one or more data center modules when the air handling module is coupled to the data center module, and wherein the one or more supply air openings and the one or more return air openings are located at the interface.