Nikolai Kondratiev was a Russian economist born in 1892, best known for his theory of long economic waves, often referred to as Kondratiev waves. He proposed that economies experience cycles of growth and decline lasting approximately 50 to 60 years, driven by technological innovations and shifts in investment patterns. Kondratiev's work gained prominence in the 1920s, but his ideas were controversial and faced criticism. He was arrested in 1930 during Stalin's regime and later executed in 1938. Despite this, his theories have influenced economic thought and are still studied today in the context of business cycles and economic forecasting.