This report reviews current research trends related to the problem of robotic exploration and mapping. In exploring and mapping an unknown environment, one fundamental problem is answering the question 'have I been here before?' (This is also known as the 'loop closing' problem.) Answering this question involves disambiguating the current place of the robot against previously visited or known locations. Two fundamental approaches to solving the problem are reviewed. The first approach resorts to the use of an 'oracle' to help solve the disambiguation problem. Oracles are available with different disambiguating powers, and the relative strengths of different oracles are explored. The second approach exploits augmenting locations with metric information that describes the underlying environment. Open questions are discussed.

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