The final report of the inquiry into the 2015 British general election opinion polls has been released.
The key finding: "the primary cause of the polling miss in 2015 was unrepresentative samples. The methods the pollsters used to collect samples of voters systematically over-represented Labour supporters and under-represented Conservative supporters. The statistical adjustment procedures applied to the raw data did not mitigate this basic problem to any notable degree."
If you don't want to read all 120 pages, Anthony Wells has a good summary here.