For Barker, networking has been essential for the success of these international collaborations.
Digital sales have also been essential for record companies to offset losses from CDs.
Those bridges would have been essential for anyone seeking higher office at Harvard.
"The efforts of frontline healthcare workers like Pauline have been essential for containing the spread of Ebola even though it means they themselves face considerable risks.
These arrests have proved particularly effective in targeting the local organisers, including student leaders who have been essential for grassroots mobilisation and co-ordination.
The Segura River valley receives water from the Tagus River through a river-diversion scheme, which has been essential for irrigation in Murcia province.
The decision was described as very disappointing by North Lancashire Citizens Advice Bureau (CAB), which said the money would have been essential for some people and could tip some vulnerable residents into rent arrears.
Nothing has vindicated that view more than the events since Sept. 11, when Russia's support has been essential for fighting the Taliban, and would be even more critical for fighting Iraq.
And doing all of those things - combined with the basic ingredients that have always been essential for effective fundraising: clear need, clear ask, clear explanation of how a donation will make a difference, loud thank you - will raise you money.
Based on our experience as an expert organisation, often working in remote areas and difficult environments (including conflict and post-conflict countries), engaging communities and strong partnerships has been essential for the success of development programmes.
In addition, Earth's atmosphere has been able to contain water in each of its three phases (solid, liquid, and gas), which has been essential for the development of life on the planet.
