Micro, small and medium-sized businesses for many years have been neglected. After World War II economy was dominated by large industrial enterprises. This was the result of the prevailing belief that factors such as labor or capital are best used in large-scale production. Only the transition to economy based on knowledge and entrepreneurship, which began in Europe in the late 90's, showed the great importance of to micro, small and medium-sized enterprises for economy. The SME sector has a very large impact on the economy, for it creates a large part of GDP, provides employment to many millions of people and is an expression of entrepreneurial activities developing in the economy. Thus the Authors attempted to analyze the SME sector in Poland after joining the European Union.