It is widely accepted that pulp fines (particles passing a 200 mesh screen) largely affect pulp properties, sheet consolidation and the final paper properties. Especially fines produced during refining – so called secondary fines- showing a more fibrillar character compared to primary fines already present after the pulping process, have a positive effect on strength properties. Although this is common knowledge within the paper physics community, it is still largely unclear which detailed properties of fines influence pulp and paper properties to what extent. As fines show some similarity to MFC, this question is also of interest regarding the sue of MFC as an additive in papermaking. We apply established and new methods for fines characterization, such as the secondary fines content, the swelling ability and data on fibrillation and fibrillary material together with a suitable experimental setup t isolate the technological impact of fines in the final product Thus we are able to evaluate the technological effect of fines with different characteristics in therms of the above mentioned properties. Our results clearly show that categorizing primary and secondary fines is not sufficient when it comes to their technological impact and only in depth analysis of the fines present in a given pulp allows to understand their effect on paper properties.