Powerful framework for running multiple operations in a pipeline, mostly replacing M/R: %$match%: for matching documents, à la %find()%. %$project%: for "rewriting" documents, but more powerful with computed expressions: adding fields (%$add%), string functions (%$toLower%), etc %$unwind%: hands out array elements each at a time %$group%: aggregates items into buckets defined by key %$sort%, %$limit%, and %$skip%