Since the 1980s, there has been a renewal of interest for organisations which are neither public nor private for-profit initiatives. These organisations are most often referred to as the "third sector". The North-American approach - that adopted by the Johns Hopkins Project (Salamon and Anheier 1997) –, which is dominant at the international level, defines this third sector as the sector grouping together all non-profit organisations. But the empirical evidence in Europe has given birth to another theoretical approach, defining the third sector as a social and solidarity economy. This contribution aims to present and explain this European conception and to compare it with the North-American one in order to stimulate the international debate.