Notes

Functional spillover: (peace by pieces spill-over) for example, the European Coal and Steel Community. With the newly integrated market for coal and steel in Europe, roads may be built to expadite transportation.
Political spillover: (getting used to brussels approval spillover) with integration, political spillover effect could be for example: centre of power shifting from the country's parliament to Brussels
Cultivated spillover: (bring everyone together spill-over) the idea that integration drives further integration by being in practice. For example, the European Union's growing autonomy. It has grown from a 6-country coal and steel market to a 28 country economic as well as political system that arguably has the most power of any institution in all of Europe. It has also instituted a common currency. The introduction of the common currency, in this case the Euro, makes many practices throughout the integrated system easier. There is no need to change currency over borders. In the European currency area (C.A. for short) currency rates or fixed in all participating countries. The downside is that it results in the loss of monetary independence and that being said, a detrimental factor or economic downturn in another country within the currency area may cause problems in your country as well.
It works well if the country is highly integrated within the region already and high mobility is a factor of production. One common currency greatly simplifies accounting for all parties throughout the Customs Union. Maximizes gains from rates without the troubles of rate fluctuation with multiple currencies.

Problem with spill-over and neofunctionalism: it assumes that integration in low politics will lead to integration in high politics. It in turn believes that areas of high politics can be affected by integration. Intergovernmentalism states that such isn't the case and policial units and the central governments have more power and that spill-over isn't a primary cause. The fate of nation-state should never be subject to the decison of others