Most international volunteering is done through placement organizations, but it is quite possible to make arrangements yourself. The benefits of doing it yourself are primarily financial. Of course there may be projects that are unavailable through a placement organization. If you have extensive knowledge of NGOs (non-governmental organizations) in a given country, you can probably do without the help of a placement organization. However, for most volunteers, the expertise of agents can be a welcome resource, increasing the odds that you will have a satisfying experience.
There are both non-profit and for-profit organizations that work in the field of volunteer placement. For-profits are not necessarily more expensive than non-profits, in fact, the opposite is frequently true. For-profits might have a "leaner" operation, or might simply pay lower salaries. Non-profits are more easily researched. They are required to provide financial information upon request. You can also access their financial statements (called a Form 990) online. There are several sites that offer this service free of charge, for example Guidestar.org.
All organizations vary somewhat in the way they structure their programs. Some provide individual placements, others provide group opportunities. For example, Volunteers for Peace specializes in international "work camps." These are short-duration, low-cost programs with a group of volunteers from many countries sent by many different organizations. ELI specializes in individual programs and tries to spread its volunteers out, frequently arranging individual homestays. This structure provides for a better immersion experience. Several large American organizations like to send groups that live together in a volunteer house, even though they might have different work projects. These are generally more highly supervised programs.
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