ISO 19901-1:2015 gives general requirements for the determination and use of meteorological and oceanographic (metocean) conditions for the design, construction and operation of offshore structures of all types used in the petroleum and natural gas industries.The requirements are divided into two broad types:- those that relate to the determination of environmental conditions in general, together with the metocean parameters that are required to adequately describe them;- those that relate to the characterization and use of metocean parameters for the design, the construction activities or the operation of offshore structures.The environmental conditions and metocean parameters discussed are:- extreme and abnormal values of metocean parameters that recur with given return periods that are considerably longer than the design service life of the structure,- long-term distributions of metocean parameters, in the form of cumulative, conditional, marginal or joint statistics of metocean parameters, and- normal environmental conditions that are expected to occur frequently during the design service life of the structure.Metocean parameters are applicable to:- the determination of actions for the design of new structures,- the determination of actions for the assessment of existing structures,- the site-specific assessment of mobile offshore units,- the determination of limiting environmental conditions, weather windows, actions and action effects for pre-service and post-service situations (i.e. fabrication, transportation and installation or decommissioning and removal of a structure), and- the operation of the platform, where appropriate.NOTE Specific metocean requirements for site-specific assessment of jack-ups are contained in ISO 19905‑1, for arctic offshore structures in ISO 19906 and for topside structures in ISO 19901‑3.