A computer-implemented method comprising: under the control of one or more computer systems configured with executable instructions, upon receiving a request to generate a virtual machine instance comprising a preference for one or more hardware specifications instantiating the virtual machine instance in a first host computer system, the first host computer system comprising one or more components that do not conform to a preference for one or more hardware specifications for instantiating a virtual machine instance, the preference for one or more hardware specifications being contained within metadata associated with the virtual machine instance; detecting a condition within a second host computer system that conforms to the preference; and migrating the virtual machine instance from the first host computer system to the second host computer system due at least in part to having detected the condition within the second host computer system upon receiving a request to generate a virtual machine instance comprising a requirement for one or more hardware specifications, wherein the one or more hardware specifications are not available displaying an error message; and, not instantiating the virtual machine instance. under the control of one or more computer systems configured with executable instructions, upon receiving a request to generate a virtual machine instance comprising a preference for one or more hardware specifications instantiating the virtual machine instance in a first host computer system, the first host computer system comprising one or more components that do not conform to a preference for one or more hardware specifications for instantiating a virtual machine instance, the preference for one or more hardware specifications being contained within metadata associated with the virtual machine instance; detecting a condition within a second host computer system that conforms to the preference; and migrating the virtual machine instance from the first host computer system to the second host computer system due at least in part to having detected the condition within the second host computer system instantiating the virtual machine instance in a first host computer system, the first host computer system comprising one or more components that do not conform to a preference for one or more hardware specifications for instantiating a virtual machine instance, the preference for one or more hardware specifications being contained within metadata associated with the virtual machine instance; detecting a condition within a second host computer system that conforms to the preference; and migrating the virtual machine instance from the first host computer system to the second host computer system due at least in part to having detected the condition within the second host computer system upon receiving a request to generate a virtual machine instance comprising a requirement for one or more hardware specifications, wherein the one or more hardware specifications are not available displaying an error message; and, not instantiating the virtual machine instance. displaying an error message; and, not instantiating the virtual machine instance.