Including guest compounds inside the pores of nanoporous crystalline hosts (e.g. zeolite) is a key strategy to post-synthetically functionalise these nanoporous materials over past half a century. It yields highly active and stable heterogeneous catalysts as well as robust materials with tuneable photoluminescence properties due to geometric/quantum confinement. More recently, metal-organic frameworks (MOFs), which are hybrid hosts assembled with metal centres and organic ligands, start to be considered for creating host-guest composites. Apart from the aforementioned confinement effects, MOFs with diverse chemistries as hosts can give rise to a variety of host-guest interactions in these composite systems. It is, however, challenging to investigate these MOF-guest systems due to small MOF pore dimensions, MOF instability, poor guest loading control and limitations in guest characterisations. The thesis explores three different MOF-guest systems covering their preparation, characterisation as well as some unusual behaviour owing to MOF-guest interactions and/or confinement effects. The first system is incorporating electrically conducting poly(3,4-ethylenedioxythiophene) (PEDOT) into a $Zn2(1,4-ndc)2(dabco)$ ($1,4-ndc=1,4-naphthalenedicarboxylate$, $dabco=1,4-diazabicyclo[2.2.2]octane$) $[PEDOT@Zn2(1,4-ndc)2(dabco)]$, which turned the electrically insulating MOF into a semiconducting composite. The formation of nanostructured PEDOT by removing the MOF from the $PEDOT@Zn2(1,4-ndc)2(dabco)$ is also demonstrated. The success in MOF-PEDOT system preparation is the motivation to expand the synthesis to forming inorganic guests (the second system). Thermodynamically favourable solid/solution phases of inorganic compounds, which relate to electrochemical potential and pH, are considered to choose reaction agents and conditions to form desired guests. The application of the concept to the synthesis of $MOF-RuO2$ catalysts ($RuO2@MOF-808-P$), where the confined $RuO2@MOF-808-P$ has exceptionally high activity for CO oxidation compared to unconfined $RuO2$ with poor activity at low temperatures ($\le$150 °C) is demonstrated. In the final system, guest-induced metamorphosis by carbonising the MOF-guest composites made of HKUST-1-type MOFs and thiomolybdates is unveiled. With implications for the transformation mechanisms, MOF- and guest-dependent morphology of the carbonised products are shown. The carbonised product based upon HKUST-1(Cu) and thiomolybdate was also demonstrated as lithium-ion battery anode.