While pair effects are referred to here as unconventional, in specific spectral domains and/or geophysical conditions they play a dominant role in the absorption/emission properties of the atmosphere, water vapour continuum absorption being one of the most prominent examples. The book clarifies still open questions in this domain and seeks to trace a path to possible answers, since the underlying phenomena are often incompletely understood and a reliable theory is often unavailable. The absence of precise laboratory data on bimolecular absorption is also often a hindrance to the construction of a reliable theoretical model. The book thus describes the latest methods, theories and techniques used to study weakly interacting molecular pairs. There is also a discussion of the serious deficiencies in our understanding of bimolecular phenomena occurring in the atmosphere that should stimulate new laboratory and theoretical investigations. The ultimate goal of the book is to bridge the gap between laboratory experiments, sophisticated theories and field observations in the interests of atmospheric science and applications.