The　ever-increasing　demand　for　flexible　and　portable　electronics　has　stimulated　research　and　development　in　building　advanced　electrochemical　energy　devices　which　are　lightweight,　ultrathin,　small　in　size,　bendable,　foldable,　knittable,　wearable,　and/or　stretchable.　In　such　flexible　and　portable　devices,　semi-solid/solid　electrolytes　besides　anodes　and　cathodes　are　the　necessary　components　determining　the　energy/power　performances.　By　serving　as　the　ion　transport　channels,　such　semi-solid/solid　electrolytes　may　be　beneficial　to　resolving　the　issues　of　leakage,　electrode　corrosion,　and　metal　electrode　dendrite　growth.　In　this　paper,　the　fundamentals　of　semi-solid/solid　electrolytes　(e.g.,　chemical　composition,　ionic　conductivity,　electrochemical　window,　mechanical　strength,　thermal　stability,　and　other　attractive　features),　the　electrode-electrolyte　interfacial　properties,　and　their　relationships　with　the　performance　of　various　energy　devices　(e.g.,　supercapacitors,　secondary　ion　batteries,　metal-sulfur　batteries,　and　metal-air　batteries)　are　comprehensively　reviewed　in　terms　of　materials　synthesis　and/or　characterization,　functional　mechanisms,　and　device　assembling　for　performance　validation.　The　most　recent　advancements　in　improving　the　performance　of　electrochemical　energy　devices　are　summarized　with　focuses　on　analyzing　the　existing　technical　challenges　(e.g.,　solid　electrolyte　interphase　formation,　metal　electrode　dendrite　growth,　polysulfide　shuttle　issue,　electrolyte　instability　in　half-open　battery　structure)　and　the　strategies　for　overcoming　these　challenges　through　modification　of　semi-solid/solid　electrolyte　materials.　Several　possible　directions　for　future　research　and　development　are　proposed　for　going　beyond　existing　technological　bottlenecks　and　achieving　desirable　flexible　and　portable　electrochemical　energy　devices　to　fulfill　their　practical　applications.　It　is　expected　that　this　review　may　provide　the　readers　with　a　comprehensive　cross-technology　understanding　of　the　semi-solid/solid　electrolytes　for　facilitating　their　current　and　future　researches　on　the　flexible　and　portable　electrochemical　energy　devices.　©　2022　American　Chemical　Society.　All　rights　reserved.