This article begins with a review of the offshore hydrocarbon industry, with emphasis on its expansion which has been especially remarkable in recent years. The average growth rate of offshore petroleum production has been 13% in the last decade, which is nearly double that of onshore production. The extensive reserves still to be discovered offshore enable us to predict that the share of offshore petroleum in worldwide production should reach 25–30% in 1980 and probably 35–40% in 1990. So as to understand this expansion better, this article goes on to examine the principal features of an offshore field. The importance of investments required for hydrocarbon discovery or production is emphasized, as is the influence of various parameters (e.g., water depth, distance from the coast) on field profitability. The low profitability rate of offshore activities up to now is mentioned, along with the fact that it has little chance of surpassing the profitability level of onshore production. After a review of these general data, the article proposes an estimate of the different expenses involved in an exploration campaign, i.e., marine geological exploration, geophysical prospecting, drilling. However, the method we describe is more for determining an order of magnitude for exploration costs than for determining unit costs of offshore operations, because such costs vary enormously with different parameters (geographical or oceanographical conditions, choice of drilling support). The article then goes on to make an estimate of the expenses involved in developing and operating a subsea field, including numerous production and development systems which are used depending on the characteristics of the field. The possible variations in investment expenditures are given, i.e., development drilling, fixed platforms, production or evacuation installations (sealine, storage tanks, terminal) along with operating expenses. To conclude; the article attempts to explain two contradictory factors in offshore operations, i.e., the large-scale expansion of exploration efforts despite the extremely high cost of all offshore production.