Are Type Ia Supernovae Standard Candles?

The use of standard candles for distance measurements is wide spread. Yet, we currently do not know a pure standard candle in astronomy. The concept of standard candles involves not only the secure establishment of a unique luminosity but also a clear observational distinction of the objects as a class. Even Type Ia supernovae, whose maximum luminosity shows amongst the smallest scatter known, need to be normalised to provide accurate distances. Without this normalisation the cosmological claims based on supernovae would not be possible. With a careful normalisation Type Ia supernovae are the best known distance indicators for cosmology to date. This is most easily shown by the small dispersion around the expansion line in the Hubble diagram. Problems with the empirical normalisation remain and a theoretical understanding of this normalisation is missing. This has direct ramifications on systematic uncertainties when deriving cosmological implications from Type Ia supernovae. Improving the understanding of supernova physics is now the prime task to sharpen this tool of observational cosmology. Once the explosion mechanism is revealed a serious discussion of possible evolutionary effects in Type Ia supernovae can start.