Secondary organic aerosol, produced by the atmospheric oxidation of volatile organic compounds, comprises a substantial portion of the total mass of atmospheric fine aerosols, but its formation mechanisms are incompletely understood. In this feature article, paying the central attention to the isoprene oxidation, whose contribution to atmospheric fine aerosol has recently been suggested, I introduce the recent developments in understanding the secondary organic aerosol formation. The aims of this feature article are (1) to explain why isoprene is previously excluded from the possible aerosol precursors, (2) to introduce the recent developments, (3) to summarize the current knowledge and (4) to elucidate the possible problems that should be discussed in the future works.