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This paper provides an early warning indicator for bubbles in financial markets. The indicator is based on traditional unit root tests, more precisely on the augmented Dickey-Fuller test and may be used in a repeated manner with rolling samples. The performance of the indicator is tested extensively via Monte Carlo simulations and comparisons of the results with the most powerful standard (stability) tests. The new indicator seems to be more robust and to have more power than the standard tests. In empirical application to US stock market data for 1871-2010, the new indicator signals most of the consensus bubbles and gives warning signals well ahead of the crash, in most cases as early as 12 months ahead. The indicator also signals most of the 'negative bubbles' before their turning points. The author would like to thank Matti Viren, Esa Jokivuolle, Jouko Vilmunen, Pentti Saikkonen, Heikki Kauppi and Ari Hyytinen for their comments at various stages of this work. I would also like to thank Nina Björklund and Tarja Yrjölä for research assistance.

The dividend yield ratio in the stock markets is, to an extent, comparable to the rent-price ratio in the housing market.Taking advantage of this definitional similarity, one can then use the traditional unit root test for log dividend yield in this case, the log rent-price ratio to test for the existence of real estate bubbles.Such unit root tests are conducted for Finland, USA, UK, Spain and Germany, and the simple test results strongly suggest the existence of bubbles in nearly all of these countries.In addition to this, we develop a continuous and monthly rent-price information-based method to track the periods when real estate prices diverge from their fundamental levels.This indicator seems to work quite well in most cases, indicating bubbles during periods which, according to the consensus literature, are seen as periods of sizable upward or downward shifts in house prices.

Tests for unit roots in log dividend yields, which are consistent with `rational bubbles' in stock prices, are conducted for the S&P500 and Finnish stock market indexes.In addition to the traditional unit root tests, we split the data into 10-year segments and use frequency domain analysis to test for the presence of unit roots in the dividend yield data.The results strongly suggest the existence of bubbles in both the US and Finnish markets.Finally we develop a novel dividend yield-based method to track periods when stock prices divert their fundamental levels. This indicator produces promising results, as it seems to have some forecasting ability concerning booms and busts in the stock markets.