This review is devoted to synthetic pathways, swelling-deswelling behavior, physico-chemical and physico-mechanical properties as well as stimuli-sensitivity of semi-interpenetrating polymer networks (SIPNs) based on crosslinked and linear polymers. The main attention is paid to systems composed of neutral or charged three-dimensional networks with embedded neutral or charged macromolecules. The peculiarities of template (co)polymerization of hydrophilic monomers in the presence of water-soluble polymers as a matrix are emphasized. Nonionic hydrogel matrixes, namely polyacrylamide, poly(acrylamideco-acrylic acid), poly(N-isopropylacrylamide) that are able to exhibit the sensitivity to dielectric permittivity, pH and temperature are mostly considered. Typical water-soluble polymers which serve as a matrix and are immobilized within networks represent the sensitive to environment water-soluble nonionic, anionic, cationic and amphoteric ones. Some examples of alginate and chitosan based SIPNs are demonstrated because these polysaccharides distinguish by commercial availability and biodegradability. Synthetic protocols of the organic-inorganic hybrid SIPN as well as hydrogel-protein SIPN are given. The SIPNs obtained by interpolyelectrolyte reactions, e.g. by interaction of ionic networks with oppositely charged linear macromolecules at gel-solution interface are also exemplified. The SIPNs containing enzymes, catalytic active functional groups, polymer-metal complexes, and nanoparticles exhibit high catalytic activity in hydrolysis, hydrogenation and decomposition of low-molecular-weight substrates. Structural, morphological, physico-chemical and physico-mechanical properties of SIPNs are determined by both network structure and nature of immobilized linear polymers. Application aspects of SIPN include drug delivery systems, pervaporation and fuel cell membranes, gel-immobilized nanosized catalysts, solar technology etc.