Primarily the financial industry. The Luxembourgish government actually actively encourages growth in this industry by providing a favorable tax climate (thus attracting banks), and by introducing favorable laws for investment funds (more laissez-faire, ie by reducing administrative procedures and auditing checks in favor of market mechanisms).
In addition, according to wikipedia (entry:economy of Luxembourg) steel and tourism also account for a major part of the economy. Also, Goodyear/Dunlop has a major production centre in Luxembourg. With over 1000 employees, and given that Luxembourg is a small country, this may possibly also be considered as "important".
Prostitution.>>> very funny the answer is TECHNOLOGY
oil and petroleum
what are the club's main source of income
what is the income of Indiana
Personal income tax
what is the main source of income in israel
Cattle
Oil
refineries
Balls
bacon
Welfare.
Luxembourg Income Study was created in 1983.
Banking, casinos and the service industry ( restaurants, stores etc) are the main sources of income.