Millisecond pulsars are extremely stable natural timekeepers. Pulsar Timing Array experiments, tracking subtle changes in the pulsars' rotation periods, can shed light on the presence of ultralight particles in our Galaxy. In this conference paper, we start by reviewing the most conservative scenario, in which ultralight particles interact only gravitationally. In this setting, we show that Pulsar Timing Arrays are able to constrain the presence of ultralight fields up to a few tenths of the observed dark matter abundance. Then, we consider conformally coupled ultralight candidates, demonstrating that the constraints on the universal scalar coupling of the field to Standard Model particles improve on existing bounds by several orders of magnitude, in the relevant mass range analyzed by Pulsar Timing Arrays. The discussion presented here is based on [1,2].