The article discusses the nature of written Baltic and Slavic sources which refer to 'Lada' and 'Ladona' (and also other forms of this theonym) and attempts to answer the question of this deity's authenticity in the pantheon of the Balts based on the work of earlier (since first half of 19th C.) investigators and ethnographic, folklore, historical and linguistic material. A portion of Lithuanian investigators of mythology have treated 'Lada' ('Ladonas') critically. Another group of investigators has harbored no doubts on the authenticity of 'Lada' (Ladonas). The opinions of investigators from other countries has also diverged on the question of the reliability of the sources, and subsequently on the authenticity of the deity 'Lada' (Ladonas). The views of N. Nikolski, T. Koleva and J. Miroliubov are discussed in detail. It has to be admitted that many investigators (and even in the second half of the 20th and beginning of the 21st century) have confined themselves to superficial and uncritical retelling or free interpretation of sources from the 15th and 16th centuries, or to the writings of mythologists and ethnographers without providing any clearer arguments witnessing to or negating the authenticity of 'Lada' (Ladonas). The author concludes that: (1) the question of the authenticity of the deities (Lyada, Dzydzilelya) referenced in the written sources of the Slavs remains unsolved; (2) in liberal anthologies of Slavic written sources, 'Lada' and 'Ladonas' are ascribed to the Balts as well. In reality the Baltic sources only provide evidence that there were rituals during the performance of which sacred songs were sung with the refrains lado, laduto; (3) uncritical usage of 16th century written sources and the works of mid-19th century ethnographers (romantics) has allowed investigators to interpret 'ledu dienos' (days of ice, hail) rituals as the adoration of winter goddess 'Lada'; (4) the refrain 'lado' (in its various forms) in the written sources of the Balts and Slavs as well as in folklore serves the function of exalting, welcoming and honoring the deity, and later also the secular ruler..