Purpose: The article aims to develop a model explaining the undertaking of Polish small enterprises' innovation activities. Design/Methodology/Approach: There are many classifications of the factors that stimulate or inhibit small enterprises' innovation activity. Generally, they are considered in two domains – as external and internal determinants. Empirical studies were conducted in 2015 on a representative sample of 202 Polish small enterprises using the CAPI method. Analysis of the significance of the impact of key internal and external factors on small enterprises' innovation activity was based on a logit regression model. Findings: The results indicate the significance of seven of the 25 key factors included in the model. These statistically significant determinants of Polish small enterprises' innovation activity exhibited both positive and negative impacts on undertaking innovative undertakings. Practical Implications: The identified determinants of innovation activity may guide managers of such companies. The presented model shows which factors stimulate the process of implementing innovations and which inhibit it. Originality/value: The research identifies the external and internal determinants of innovation activity in Polish small companies. These results can be compared with results in other countries, where the conditions for conducting innovation activities are often completely different.