Maria Cristina Baglio, INAF, Observatory of Brera, Merate, Italy — February 5, 2024

In this review, I will provide a comprehensive overview of transitional millisecond pulsars from an observational perspective. These objects earn the label 'transitional' due to their ability to switch back and forth between X-ray and radio pulsar states in a few days timescale, in response to variations of the rate of mass accretion.
I'll delve into the multi-wavelength signatures observed during both the X-ray and radio pulsar states, enabling us to investigate distinct emission mechanisms active in various evolutionary phases. Subsequently, I will present novel findings derived from observations of the prototypical transitional pulsar, PSR J1023+0038, emphasizing the recent discovery of pulsations in the optical band. Furthermore, I will explore the most recent model that attempts to explain the origin of the puzzling behaviour of these sources while in their X-ray state, shedding light on the latest advancements in this field and outlining the lingering questions that await resolution.