The article reviews the theoretical and practical aspects of polymer-polymer miscibility in the solid amorphous state, focusing on homopolymers and both random and block copolymers. Despite the random mixing hypothesis being a common feature of most theoretical treatments, many observed single-phase polymer-polymer mixtures involve specific interactions such as hydrogen bonding. The composition of random copolymers can often be tailored to achieve miscibility with specific homopolymers. Many polymer-polymer mixtures exhibit lower critical solution temperatures, and a few show upper critical solution temperatures. The article discusses the special phenomena observed when other polymers are mixed with block copolymers. The Flory-Huggins theory is applied to predict the free energy of mixing, but it does not account for specific interactions. The article also explores the effects of molecular weight polydispersity and the peculiarities of block copolymers. Experimental methods for determining polymer-polymer miscibility, including visual and microscopic observations, glass transition temperatures, and crystalline melting points, are discussed. The literature data on miscible polymer pairs, including those with specific interactions and those with specific compositions, are summarized, highlighting the importance of specific interactions and composition in achieving miscibility.The article reviews the theoretical and practical aspects of polymer-polymer miscibility in the solid amorphous state, focusing on homopolymers and both random and block copolymers. Despite the random mixing hypothesis being a common feature of most theoretical treatments, many observed single-phase polymer-polymer mixtures involve specific interactions such as hydrogen bonding. The composition of random copolymers can often be tailored to achieve miscibility with specific homopolymers. Many polymer-polymer mixtures exhibit lower critical solution temperatures, and a few show upper critical solution temperatures. The article discusses the special phenomena observed when other polymers are mixed with block copolymers. The Flory-Huggins theory is applied to predict the free energy of mixing, but it does not account for specific interactions. The article also explores the effects of molecular weight polydispersity and the peculiarities of block copolymers. Experimental methods for determining polymer-polymer miscibility, including visual and microscopic observations, glass transition temperatures, and crystalline melting points, are discussed. The literature data on miscible polymer pairs, including those with specific interactions and those with specific compositions, are summarized, highlighting the importance of specific interactions and composition in achieving miscibility.