The unresolved debate on the existence of weather cycles is explored in detail within this book. The evidence for and against the existence of cycles in the weather is examined and the difficulties in establishing the credentials of cycles discussed. The competing arguments involve the natural variability of the climate, the influence of sunspots and the variations in the Earth's orbit. A wide range of events from the ice ages to the El Nino and many examples of apparently cyclic behaviour are presented, with evidence from both instrumental observations and other records such as tree rings, ice cores and ocean sediments. These data are set in the context of statistical analysis, global climatology and the predictability of complex non-linear systems (Chaos Theory). The general conclusion is that, with few exceptions, the case for cycles is not proven, but that further work on the origin of long term fluctuations in the weather is essential to the understanding of the current changes in the Earth's climate. The complex analysis needed to look at the evidence for cycles is examined with the minimum of mathematics, so that this book will appeal to readers with an interest in how the weather works, as well as to the workers in this important and controversial field.