A method for estimating wear damage of a rolling-element bearing system comprising at least one rolling-element bearing, the method comprising: generating a first number of first condition indicators representative of the wear damage of the at least one rolling-element bearing, wherein the first number of first condition indicators comprises a first set of fluid debris indicators and a first set of vibration indicators, wherein the first set of fluid debris indicators are generated based on monitoring a flow of fluid in fluid communication with the at least one rolling-element bearing and the first set of vibration indicators are generated based on monitoring vibrations associated with the at least one rolling-element bearing; generating a second number of second condition indicators based on the first number of first condition indicators, wherein the second number is less than the first number as: at least one of the second condition indicators is based upon a sum of a function of a mass of oil debris, a function of an oil debris count and a function of an oil debris mass rate, and at least one of the second condition indicators is based upon vibration data from the vibration indicators based upon which of the at least one rolling-element bearing the vibration data is relevant thereto, the vibration data being grouped and then normalized; and generating an indication of the wear damage of the at least one rolling-element bearing based on the second number of second condition indicators. generating a first number of first condition indicators representative of the wear damage of the at least one rolling-element bearing, wherein the first number of first condition indicators comprises a first set of fluid debris indicators and a first set of vibration indicators, wherein the first set of fluid debris indicators are generated based on monitoring a flow of fluid in fluid communication with the at least one rolling-element bearing and the first set of vibration indicators are generated based on monitoring vibrations associated with the at least one rolling-element bearing; wherein the first number of first condition indicators comprises a first set of fluid debris indicators and a first set of vibration indicators, wherein the first set of fluid debris indicators are generated based on monitoring a flow of fluid in fluid communication with the at least one rolling-element bearing and the first set of vibration indicators are generated based on monitoring vibrations associated with the at least one rolling-element bearing; generating a second number of second condition indicators based on the first number of first condition indicators, wherein the second number is less than the first number as: at least one of the second condition indicators is based upon a sum of a function of a mass of oil debris, a function of an oil debris count and a function of an oil debris mass rate, and at least one of the second condition indicators is based upon vibration data from the vibration indicators based upon which of the at least one rolling-element bearing the vibration data is relevant thereto, the vibration data being grouped and then normalized; and at least one of the second condition indicators is based upon a sum of a function of a mass of oil debris, a function of an oil debris count and a function of an oil debris mass rate, and at least one of the second condition indicators is based upon vibration data from the vibration indicators based upon which of the at least one rolling-element bearing the vibration data is relevant thereto, the vibration data being grouped and then normalized; generating an indication of the wear damage of the at least one rolling-element bearing based on the second number of second condition indicators.