This thesis compares different sources of information about sea ice off the coasts of Iceland and presents new indices of sea ice severity for the years 1901-1925. The sea around Iceland is divided into eighteen Zones, each of which is given an icecode, referring to the amount of sea ice that was observed in each month. This information is tabulated and the reliability of the data sources is indicated. Further work with this data includes a description of the frequency of ice in each zone, year and month. The indices are compared with mean annual temperature in Stykkisholmur, W-Iceland, and with temperature anomalies for the whole Northern Hemisphere. The effects which the sea ice had in Iceland in this period are discussed.