  We present a historical review of Einstein's 1917 paper 'Cosmological
Considerations in the General Theory of Relativity' to mark the centenary of a
key work that set the foundations of modern cosmology. We find that the paper
followed as a natural next step after Einstein's development of the general
theory of relativity and that the work offers many insights into his thoughts
on relativity, astronomy and cosmology. Our review includes a description of
the observational and theoretical background to the paper; a
paragraph-by-paragraph guided tour of the work; a discussion of Einstein's
views of issues such as the relativity of inertia, the curvature of space and
the cosmological constant. Particular attention is paid to little-known aspects
of the paper such as Einstein's failure to test his model against observation,
his failure to consider the stability of the model and a mathematical oversight
concerning his interpretation of the role of the cosmological constant. We
recall the response of theorists and astronomers to Einstein's cosmology in the
context of the alternate models of the universe proposed by Willem de Sitter,
Alexander Friedman and Georges Lemaitre. Finally, we describe the relevance of
the Einstein World in today's 'emergent' cosmologies.
