Some Remarks on Alternative (or Modified) Theories of Gravity

A seminar given about 30 years ago by Ruben Aldrovandi motivates this text where some reflexions about constructing theories that modify General Relativity are made. Two particular cases, the Brans-Dicke and Unimodular Gravity ones, are discussed, in a quite qualitative way, showing on how they can address some of the most outstanding problems of General Relativity, specially the transplanckian physics and the cosmological constant problem.