ToF-SIMS is a surface-sensitive mass spectrometric technique which combines high sensitivity and mass resolution (m/delta m 5000 to 10,000) with the capability of obtaining atomic and molecular information (mass range 1-10,000 micron) in 2 dimensions, and even 3 dimensions if a focused ion beam is used. Therefore ToF-SIMS is a promising tool for studying small amounts of organic and inorganic material in-situ in geological samples. In this talk I will describe the technique and show a number different applications of this technique for both modern and ancient samples with a focus on organic biomarkers trapped in oil-bearing fluid inclusions. Oil-bearing fluid inclusions preserve some the oldest uncontaminated record of organic biomarkers such as hopanes and steranes. Information obtained from these samples may therefore answer some key questions regarding the early evolution of life on Earth. However, most inclusions, including Precambrian oil-bearing fluid inclusions are so small (less than 10 µm) and few that analyzing them with conventional techniques are not possible. Therefore, we have developed an approach employing ToF-SIMS to selectively open individual oil-bearing inclusions by C60+ ion etching, and to subsequently analyse their content. Using this approach steranes and hopanes could be detected in single Ordovician oil-bearing inclusions (15-30 µm) from the Siljan impact structure in Sweden. When applied on Precambrian samples we were able to detect steranes and hopanes in Proterozoic sandstones from Australia indicating the presence of eukaryotes at least 1.43 Ga ago.