Continual improvement is a key component of environmental management systems (EMS) according to ISO 14001 standard. But in contrast to most other specifications in the standard, it is difficult to assess. The following article discusses factors influencing the EMS improvement in Polish companies according to ISO 14001 standard. Research of the factors influencing the EMS improvement was conducted in Polish companies from July 2007 to April 2009 by the Department of Standardized Management Systems at the Poznań University of Economics. The research was carried out at 1,360 companies that have certified their Environmental Management System against ISO 14001 requirements. There is a significant number of factors that can influence improvement of the EMS and there are many elements that can be improved in it.