Monte Carlo methods are extensively used in Nuclear Medicine to tackle a variety of problems that are difficult to study by an experimental or analytical approach. A review of the most recent tools allowing application of Monte Carlo methods in single photon emission tomography (SPET) and positron emission tomography (PET) is presented. To help potential Monte Carlo users choose a code, we present advantages and disadvantages of the different types of Monte Carlo codes currently available for SPET and PET, discuss common and specific features of the codes, classify the codes with respect to these features, comment key properties for a code to be appropriate for a given purpose and, at last, we consider the possibility of going towards a standardisation of the description of the codes which could facilitate their comparison.